Update CHANGELOG.md for 12.9.0

[ci skip]

changelogs/unreleased/sh-avoid-current-settings-rake-task.yml

----
-title: Only load usage ping cron schedule for Sidekiq
-merge_request: 25325
-author:
-type: other

changelogs/unreleased/sh-cache-ci-variables.yml

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-title: Memoize loading of CI variables
-merge_request: 26147
-author:
-type: performance

changelogs/unreleased/sh-cleaup-prom-background-migrations.yml

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-title: Clean stale background migration jobs
-merge_request: 25707
-author:
-type: fixed

changelogs/unreleased/sh-enable-redis-key-compression.yml

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-title: Enable Redis cache key compression
-merge_request: 27254
-author:
-type: performance

changelogs/unreleased/sh-ensure-fresh-project-settings.yml

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-title: Ensure freshness of settings with project creation
-merge_request: 27156
-author:
-type: fixed

changelogs/unreleased/sh-feature-flag-persisted-process-cache.yml

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-title: Move feature flag list into process cache
-merge_request: 27511
-author:
-type: performance

changelogs/unreleased/sh-optimize-pipeline-for-mrs.yml

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-title: Fix N+1 queries for PipelinesController#index.json
-merge_request: 26643
-author:
-type: performance

changelogs/unreleased/sh-rate-limit-archive-endpoint.yml

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-title: Rate limit archive endpoint by user
-merge_request: 25750
-author:
-type: changed

changelogs/unreleased/sh-refresh-mr-widget-upon-cancel.yml

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-title: Refresh widget after canceling "Merge When Pipeline Succeeds"
-merge_request: 26232
-author:
-type: fixed

changelogs/unreleased/sh-upgrade-bootsnap-1-4-6.yml

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-title: Upgrade to Bootsnap 1.4.6
-merge_request: 25844
-author:
-type: performance

changelogs/unreleased/sh-use-process-cache-for-feature-flags.yml

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-title: Use process-wide memory cache for feature flags
-merge_request: 26935
-author:
-type: performance

changelogs/unreleased/sha-params-validator.yml

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-title: Add grape custom validator for sha params
-merge_request: 26220
-author: Rajendra Kadam
-type: added

changelogs/unreleased/show-cluster-status-fe.yml

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-title: Show cluster status (FE)
-merge_request: 26368
-author:
-type: added

changelogs/unreleased/sidebar_not_expanding_at_certain_resolutions.yml

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-title: Fix bug with sidebar not expanding at certain resolutions
-merge_request: 25313
-author: Lee Tickett
-type: fixed

changelogs/unreleased/simplify-ide-colors.yml

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-title: Simplifying colors in the Web IDE
-merge_request: 25304
-author:
-type: other

changelogs/unreleased/stop_environments.yml

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-title: 'Fixes stop_review job upon expired artifacts from previous stages'
-merge_request: 27258
-author: Jack Lei
-type: fixed

changelogs/unreleased/support-airgap-in-dependency-scanning-template.yml

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-title: Add airgap support to Dependency Scanning template
-merge_request: 26145
-author:
-type: changed

changelogs/unreleased/switch-ff-ci-dynamic-child-pipeline.yml

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-title: Enable feature Dynamic Child Pipeline creation via artifact
-merge_request: 26648
-author:
-type: added

changelogs/unreleased/sy-alert-embeds.yml

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-title: Add support for alert-based metric embeds in GFM
-merge_request: 25075
-author:
-type: added

changelogs/unreleased/sy-auto-embed-alert.yml

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-title: Automatically include embedded metrics for GitLab alert incidents
-merge_request: 25277
-author:
-type: added