Standardize upgrade path anchor links in update docs

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Problem

Internal anchor links in doc/update/upgrade_paths.md use outdated #upgrades-to-* format that doesn't match actual heading anchors in target files. This creates broken navigation and inconsistent linking patterns.

What needs to be done

Standardize anchor links to match GitLab's auto-generated heading anchors:

Verified Changes Required

• Line 35: Update GitLab 18 anchor from #upgrades-to-XXXX to #gitlab-18
• Lines 47-50: GitLab 17 anchors already correct (#17110, #1780, #1750, #1730)

Verification Completed

Cross-referenced with live rendered docs:

• https://docs.gitlab.com/ee/update/versions/gitlab_18_changes.html#gitlab-18
• https://docs.gitlab.com/ee/update/versions/gitlab_17_changes.html

Confirmed anchor generation rules:

• Headings like "## GitLab 18" → #gitlab-18
• Version headings like "## 17.11.0" → #17110 (dots stripped)

Acceptance Criteria

• Line 35: Update to [upgrade notes for GitLab 18](versions/gitlab_18_changes.md#gitlab-18)

MR Guidance

Branch: git checkout -b standardize-upgrade-anchor-links

Files to edit: doc/update/upgrade_paths.md (line 35 only)

Testing: Preview docs and click-test updated links

Implementation Notes

This is a targeted fix based on comprehensive verification. Lines 47-50 are already correct and require no changes.