Update CHANGELOG.md for 12.6.0

[ci skip]

changelogs/unreleased/21800-mentioned-users-table-migrations.yml

----
-title: Creates DB tables for storing mentioned users, groups, projects referenced
-  in a note or issuable description
-merge_request: 18316
-author:
-type: added

changelogs/unreleased/22392-remove-eks-cluster-feature-flag.yml

----
-title: Enable creating Amazon EKS clusters from GitLab
-merge_request: 20333
-author:
-type: added

changelogs/unreleased/22392-update-cluster-settings-provider-link.yml

----
-title: Update external link to provider in cluster settings
-merge_request: 20425
-author:
-type: fixed

changelogs/unreleased/22465-rack-attack-authenticate-runner-requests-with-job-token-basic-auth.yml

----
-title: Authenticate requests with job token as basic auth header for request limiting
-merge_request: 21562
-author:
-type: fixed

changelogs/unreleased/22465-rack-attack-authenticate-runner-requests.yml

----
-title: Authenticate runner requests in Rack::Attack
-merge_request: 21311
-author:
-type: fixed

changelogs/unreleased/23746-error-500-when-trying-to-destroy-oauth-application-entry-in-admin-a.yml

----
-title: Use cascading deletes for deleting oauth_openid_requests upon deleting an oauth_access_grant
-merge_request: 19617
-author:
-type: fixed

changelogs/unreleased/24190-archived-project-warning-message.yml

----
-title: Show correct warning on issue when project is archived
-merge_request: 20078
-author:
-type: fixed

changelogs/unreleased/24804-import-page-namespace-fix.yml

----
-title: Fixed project import from export ignoring namespace selection
-merge_request: 20405
-author:
-type: fixed

changelogs/unreleased/25351-texts-and-structure.yml

----
-title: Update information and button text for deployment footer
-merge_request: 18918
-author:
-type: changed

changelogs/unreleased/26013-release-generation-from-within-gitlab-ci-yml-6.yml

----
-title: Allow Job-Token authentication on Releases creation API
-merge_request: 20632
-author:
-type: added

changelogs/unreleased/26019-add-evidence-json.yml

----
-title: Add evidence collection for Releases
-merge_request: 18874
-author:
-type: changed

changelogs/unreleased/26019-restriction-on-evidence.yml

----
-title: Evidence - Added restriction for guest on Release page
-merge_request: 21102
-author:
-type: changed

changelogs/unreleased/26293-prevent-merge-if-pipeline-has-failed.yml

----
-title: Update merging an MR behavior on the API when pipeline fails
-merge_request: 19641
-author: briankabiro
-type: fixed

changelogs/unreleased/27025-add-border-top-in-readme.yml

----
-title: Fix top border of README file header in file list
-merge_request: 21314
-author:
-type: fixed

changelogs/unreleased/27239-display-a-better-message-when-starting-a-discussion-on-a-deleted-co.yml

----
-title: Display a better message when starting a discussion on a deleted comment
-merge_request: 20031
-author: Jacopo Beschi @jacopo-beschi
-type: changed

changelogs/unreleased/27244-discard-all-changes.yml

----
-title: Fix "Discard all" for new and renamed files
-merge_request: 21854
-author:
-type: fixed

changelogs/unreleased/27244-webide-discard-all-changes.yml

----
-title: "Web IDE: Fix the console error that happens when discarding a newly added/uploaded file."
-merge_request: 21537
-author:
-type: fixed

changelogs/unreleased/27630-deploy-to-ci-specified-namespace.yml

----
-title: Use CI configured namespace for deployments to unmanaged clusters
-merge_request: 20686
-author:
-type: added

changelogs/unreleased/27630-fail-build-if-namespace-specified-in-ci-for-managed-cluster.yml

----
-title: Do not allow specifying a Kubernetes namespace via CI template for managed
-  clusters
-merge_request: 21223
-author:
-type: added

changelogs/unreleased/27630-specify-kubernetes-namespace-in-ci-template.yml

----
-title: Allow specifying Kubernetes namespace for an environment in gitlab-ci.yml
-merge_request: 20270
-author:
-type: added