Enable task list in milestone show view

What does this MR do and why?

• This MR fixes the issue Task lists in Milestone description are disable... (#27338 - closed)
• With this MR, we are now able to toggle the task list items in the milestone description directly from the milestone detail view => we do not need to go the milestone edit page

🛠 with ❤ at Siemens

Screenshots or screen recordings

The following screencast shows the difference between the current version on gitlab.com (left-hand side) and my local dev machine (right-hand side).

2023-04-27-gitlab-milestone-edit-task-list-in-description-low

We also included an error message when task list items have been updated concurrently, see screenshot below

How to set up and validate locally

1. Go to a milestone, e.g. http://127.0.0.1:3000/gitlab-instance-6ede5dc7/Monitoring/-/milestones/1
2. Edit and save a description that contains some task list items, e.g.:

- [x] Task 1 (completed)
- [ ] Task 2 (open)
- [ ] Task 3 (play)

3. Start toggling the task list items <= this is not possible in the current version
4. Check if the description has been updated respectively by going to the edit view again

TODO

• @gerardo-navarro Implement callback route for milestone, see Project::MergeRequestController#update
• @gerardo-navarro Create new issue for implementing the field lock_version for the model Milestone
• @gerardo-navarro Implement tests for milestone controllers as I needed to adjust them as well
• @gerardo-navarro Implement feature tests
• Implement migration lock_version, see Add lock_version to milestone (!119859 - merged)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

• Changelog entry added, if necessary

• Documentation created/updated via this MR

• Documentation reviewed by technical writer or follow-up review issue created

• Tests added for this feature/bug

• Tested in all supported browsers

• Conforms to the code review guidelines

• Conforms to the merge request performance guidelines

• Conforms to the style guides

• Conforms to the javascript style guides

• Conforms to the database guides

• Depends on the following MR, Add lock_version to milestone (!119859 - merged)

Related to #27338 (closed)