Update CHANGELOG.md for 12.9.0

[ci skip]

changelogs/unreleased/use-default-crossplane-stack-versions.yml

----
-title: Fix installation of GitLab-managed crossplane chart
-merge_request: 27040
-author:
-type: fixed

changelogs/unreleased/use_replicate_repo_for_repo_move.yml

----
-title: Use ReplicateRepository when moving repo storage
-merge_request: 26550
-author:
-type: changed

changelogs/unreleased/validate-subnets-field.yml

----
-title: Validate that users selects at least two subnets in EKS Form
-merge_request: 26936
-author:
-type: fixed

changelogs/unreleased/validate_service_project_id_nil_if_template.yml

----
-title: Validate absence of project_id if service is a template
-merge_request: 26563
-author:
-type: other

changelogs/unreleased/vh-snippets-content-types.yml

----
-title: Remove unused Snippets#content_types method
-merge_request: 26306
-author:
-type: other

changelogs/unreleased/vh-snippets-finder-tweak.yml

----
-title: Improve SnippetsFinder performance with disabled project snippets
-merge_request: 26295
-author:
-type: performance