Update CHANGELOG.md for 12.6.0

[ci skip]

changelogs/unreleased/33596-package-ci-build-data.yml

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-title: Add Pipeline Metadata to Packages
-merge_request: 19796
-author:
-type: added

changelogs/unreleased/33596-package-ci-metadata.yml

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-title: Improve job tokens and provide access helper
-merge_request: 19793
-author:
-type: other

changelogs/unreleased/33718-add-new-dep-scanning-flag.yml

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-title: Add dependency scanning flag for skipping automatic bundler audit update
-merge_request: 20743
-author:
-type: added

changelogs/unreleased/33880-add-api-to-disable-pages.yml

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-title: Add API endpoint to unpublish GitLab Pages
-merge_request: 19781
-author:
-type: added

changelogs/unreleased/34067-add-recent-searches-to-sentry-error-list-in-gitlab.yml

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-title: Add recent search to error tracking
-merge_request: 19301
-author:
-type: added

changelogs/unreleased/34068-sort-by-first-seen-last-seen-and-frequency-in-sentry-error-list-in-.yml

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-title: Add sort param to error tracking issue index
-merge_request: 20101
-author:
-type: changed

changelogs/unreleased/34068-sort-error-list.yml

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-title: Sort Sentry error list by first seen, last seen or frequency
-merge_request: 21250
-author:
-type: added

changelogs/unreleased/34121-add-error-states-to-getters.yml

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-title: Add specific error states to dashboard
-merge_request: 21618
-author:
-type: added

changelogs/unreleased/34121-group-level-no-data-store.yml

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-title: Add empty region when group metrics are missing
-merge_request: 20900
-author:
-type: fixed

changelogs/unreleased/34157-apm_snowplow_events.yml

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-title: Add snowplow events for APM
-merge_request: 19463
-author:
-type: added

changelogs/unreleased/34157-snowplow-custom-events-for-monitor-apm.yml

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-title: Add snowplow events for monitoring dashboard
-merge_request: 19455
-author:
-type: added

changelogs/unreleased/34261-service-desk-to-api.yml

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-title: Add service desk information to projects API endpoint
-merge_request: 20913
-author:
-type: changed

changelogs/unreleased/34261-service-desk-to-graphql.yml

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-title: Add service desk information to project graphQL endpoint
-merge_request: 20722
-author:
-type: changed

changelogs/unreleased/34261-toggle-service-desk.yml

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-title: Support toggling service desk from API
-merge_request: 21627
-author:
-type: changed

changelogs/unreleased/34377-design-view-download-single-issue-design-image.yml

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-title: 'Resolve Design view: Download single issue design image'
-merge_request: 20703
-author:
-type: added

changelogs/unreleased/34406-link-to-merge-trains-documentation-on-mr-widget-is-incorrect.yml

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-title: Correct link to Merge trains documentation on MR widget
-merge_request: 19726
-author:
-type: changed

changelogs/unreleased/34457-remove-n-plus-1-search.yml

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-title: Remove an N+1 call rendering projects search results
-merge_request: 21626
-author:
-type: performance

changelogs/unreleased/34636-drop-deprecated-column-from-projects-table.yml

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-title: Drop deprecated column from projects table
-merge_request: 18914
-author:
-type: deprecated

changelogs/unreleased/34685-Pages-template-jekyll-outdated-and-not-working-as-expected.yml

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-title: Updated jekyll project_template
-merge_request: 20090
-author: Marc Schwede
-type: other

changelogs/unreleased/34734-monitor-dashboard-does-not-expand-sections-that-have-data-on-load.yml

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-title: Fix graph groups in monitor dashboard that are hidden on load
-merge_request: 20312
-author:
-type: fixed