Update CHANGELOG.md for 12.9.0

[ci skip]

changelogs/unreleased/sy-global-integration.yml

----
-title: Display GitLab issues created via Sentry global integration
-merge_request: 26418
-author:
-type: fixed

changelogs/unreleased/sy-grafana-default-panel.yml

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-title: Default to first valid panel in unspecified Grafana embeds
-merge_request: 21932
-author:
-type: changed

changelogs/unreleased/sy-grafana-default-times.yml

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-title: Allow default time window on grafana embeds
-merge_request: 21884
-author:
-type: changed

changelogs/unreleased/tokenize-filtered-search.yml

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-title: Use colon to tokenize input in filtered search
-merge_request: 26072
-author:
-type: changed

changelogs/unreleased/turn-on-new-variables-ui-ff.yml

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-title: Update UI for project and group settings CI variables
-merge_request: 26901
-author:
-type: added

changelogs/unreleased/tweak-wiki-title-validation-message.yml

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-title: Include invalid directories in wiki title message
-merge_request: 25376
-author:
-type: changed

changelogs/unreleased/udpate-cluster-application-image-to-0-11.yml

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-title: Update cluster-applications image to v0.11 with a runner bugfix, updated cert-manager,
-  and vault as a new app
-merge_request: 26842
-author:
-type: changed

changelogs/unreleased/udpate-cluster-application-image-to-0-12.yml

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-title: Adds crossplane as CI/CD Managed App
-merge_request: 27374
-author:
-type: added

changelogs/unreleased/unique-service-template-per-type.yml

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-title: Validates only one service template per type
-merge_request: 26380
-author:
-type: other

changelogs/unreleased/unlink-cache-deletions.yml

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-title: Swap to UNLINK for Redis set cache
-merge_request: 27116
-author:
-type: performance

changelogs/unreleased/update-ado-image-to-0-10-0.yml

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-title: Update Auto DevOps deployment template's auto-deploy-image to v0.10.0 (updates the included glibc)
-merge_request: 25920
-author:
-type: other

changelogs/unreleased/update-cert-manager-to-0-10-1.yml

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-title: Use cert-manager 0.10 instead of 0.9 for new chart installations
-merge_request: 26345
-author:
-type: changed

changelogs/unreleased/update-dast-ado-image-to-0-10-0.yml

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-title: Update DAST auto-deploy-image to v0.10.0
-merge_request: 25922
-author:
-type: other

changelogs/unreleased/update-gitlab-runner-helm-chart-to-0-14-0.yml

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-title: Update GitLab Runner Helm Chart to 0.14.0
-merge_request: 25749
-author:
-type: other

changelogs/unreleased/update-private-project-wording.yml

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-title: Clarify private visibility for projects.
-merge_request: 25852
-author:
-type: other

changelogs/unreleased/update-puma-to-4-3-3.yml

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-title: Update Puma to 4.3.3
-merge_request: 27232
-author:
-type: security

changelogs/unreleased/update-ruby-version-on-official-ci-templates.yml

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-title: Update Ruby version in official CI templates
-merge_request: 23585
-author: Takuya Noguchi
-type: other

changelogs/unreleased/update_ingress_chart_version.yml

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-title: 'Update Ingress chart version to 1.29.7'
-merge_request: 25949
-author:
-type: added

changelogs/unreleased/update_repo_storage_checksum.yml

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-title: Ensure checksums match when updating repository storage
-merge_request: 26334
-author:
-type: changed

changelogs/unreleased/upgrade-gitlab-ui.yml

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-title: Remove special chars from previous and next items in pagination
-merge_request: 25891
-author:
-type: other