Update parent link validate_max_children validation

What does this MR do and why?

Related to #451062 (closed)

When migrating epics to work items some parent epics could have more children than the limit set for parent links (100) but we need these links to remain valid after the backfilling. To achieve it, this MR modifies the validation validate_max_children in WorkItems::ParentLink model to be performed only when the parent changes so this will cover both create and update actions.

As these changes make the validation more lenient there is no risk involved for existing records.

Changelog: changed

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Screenshots or screen recordings

How to set up and validate locally

1. Enable feature flag Feature.enable(:namespace_level_work_items)
2. Visit any group work items URL and create a work item epic Parent Epic. E.g. http://gdk.test:3000/groups/GROUP_PATH/-/work_items/new
3. In the child items create and add 2 new child issues
4. Go to your IDE and open the file app/models/work_items/parent_link.rb to modify the MAX_CHILDREN variable. Set a limit of 1.
5. Back in the browser try to add a new issue to Parent Epic, this should display an error because the limit has been reached
6. Verify that the existing 2 children are valid by changing their position in the list (when invalid the update action for reordering would fail)

assigned to @egrieff

added backend groupproduct planning sectiondev typemaintenance labels

added devopsplan label

added typefeature label and removed typemaintenance label

	2 Warnings
	b9b71d7e: The commit body should not contain more than 72 characters per line. For more information, take a look at our Commit message guidelines.
	This merge request does not refer to an existing milestone.

Reviewer roulette

Category	Reviewer	Maintainer
backend	@syarynovskyi (UTC+3, 1 hour ahead of author)	@lma-git (UTC-7, 9 hours behind author)

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changed the description

@akotte Would you mind taking the first review, please?

requested review from @akotte

approved this merge request

added pipeline:mr-approved label

@akotte, thanks for approving this merge request.

This is the first time the merge request has been approved. To ensure we don't only run predictive pipelines, and we don't break master, a new pipeline will be started shortly.

Please wait for the pipeline to start before resolving this discussion and set auto-merge for the new pipeline. See merging a merge request for more details.

requested review from @engwan

removed review request for @akotte

E2E Test Result Summary

allure-report-publisher generated test report!

e2e-test-on-gdk: test report for b9b71d7e

expand test summary

+------------------------------------------------------------------+
|                          suites summary                          |
+-------------+--------+--------+---------+-------+-------+--------+
|             | passed | failed | skipped | flaky | total | result |
+-------------+--------+--------+---------+-------+-------+--------+
| Plan        | 51     | 0      | 2       | 47    | 53    | ✅     |
| Verify      | 31     | 0      | 1       | 30    | 32    | ✅     |
| Package     | 19     | 0      | 12      | 19    | 31    | ✅     |
| Create      | 101    | 0      | 9       | 94    | 110   | ✅     |
| Analytics   | 2      | 0      | 0       | 1     | 2     | ✅     |
| Govern      | 66     | 0      | 0       | 43    | 66    | ✅     |
| Release     | 5      | 0      | 0       | 5     | 5     | ✅     |
| Monitor     | 7      | 0      | 0       | 7     | 7     | ✅     |
| Data Stores | 31     | 0      | 0       | 22    | 31    | ✅     |
| Manage      | 0      | 0      | 1       | 0     | 1     | ➖     |
+-------------+--------+--------+---------+-------+-------+--------+
| Total       | 313    | 0      | 25      | 268   | 338   | ✅     |
+-------------+--------+--------+---------+-------+-------+--------+

e2e-package-and-test: test report for b9b71d7e

expand test summary

+------------------------------------------------------------------+
|                          suites summary                          |
+-------------+--------+--------+---------+-------+-------+--------+
|             | passed | failed | skipped | flaky | total | result |
+-------------+--------+--------+---------+-------+-------+--------+
| Plan        | 285    | 0      | 23      | 0     | 308   | ✅     |
| Create      | 183    | 0      | 20      | 0     | 203   | ✅     |
| Verify      | 10     | 0      | 0       | 0     | 10    | ✅     |
| Govern      | 28     | 0      | 0       | 0     | 28    | ✅     |
| Package     | 6      | 0      | 8       | 0     | 14    | ✅     |
| Data Stores | 22     | 0      | 0       | 0     | 22    | ✅     |
| Monitor     | 8      | 0      | 0       | 0     | 8     | ✅     |
| Release     | 2      | 0      | 0       | 0     | 2     | ✅     |
+-------------+--------+--------+---------+-------+-------+--------+
| Total       | 544    | 0      | 51      | 0     | 595   | ✅     |
+-------------+--------+--------+---------+-------+-------+--------+

approved this merge request

removed review request for @engwan

enabled an automatic merge when all merge checks for b9b71d7e pass

resolved all threads

requested review from @engwan

changed milestone to %17.0

merged

mentioned in commit 758d0385

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added releasedcandidate label

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mentioned in issue #463557 (closed)

mentioned in merge request kubitus-project/kubitus-installer!3058 (merged)

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