Update CHANGELOG.md for 12.0.0

[ci skip]

changelogs/unreleased/upgrade-pages-to-v1-6-1.yml

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-title: Update GitLab Pages to v1.6.1
-merge_request: 29559
-author:
-type: other

changelogs/unreleased/variables-boolean-type.yml

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-title: 'API: change masked attribute type to Boolean'
-merge_request: 28758
-author:
-type: other

changelogs/unreleased/visual-review-apps-fix-dropdown.yml

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-title: Move Dropdown to Stick to MR View App Button
-merge_request: 29767
-author:
-type: fixed

changelogs/unreleased/weimeng-email-routing.yml

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-title: Add ability to define notification email addresses for groups you belong to.
-merge_request: 25299
-author:
-type: added

changelogs/unreleased/winh-notes-error-handling.yml

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-title: Handle errors in successful notes reply
-merge_request: 28082
-author:
-type: fixed

changelogs/unreleased/zj-remove-delta-island-feature-flag.yml

-merge_request: 28871
-title: Improve clone performance by using delta islands
-type: performance

changelogs/unreleased/zj-usage-ping-pool-repository.yml

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-title: Add Pool repository to the usage ping
-merge_request: 28267
-author:
-type: other