Update CHANGELOG.md for 12.6.0

[ci skip]

changelogs/unreleased/27630-use-ci-specified-namespace-for-deploy-boards.yml

----
-title: Allow Kubernetes namespaces specified via CI template to be used for terminals,
-  pod logs and deploy boards
-merge_request: 21460
-author:
-type: added

changelogs/unreleased/28075-error-while-fetching-envs.yml

----
-title: Added lightweight check when retrieving Prometheus metrics.
-merge_request: 21099
-author:
-type: performance

changelogs/unreleased/28328-lock-button-fix.yml

----
-title: Unlock button changed from Icon to String
-merge_request: 20307
-author:
-type: changed

changelogs/unreleased/28365-bypass-confirmation-box-for-delete-user-and-contributions.yml

----
-title: Fix delete user dialog bypass caused by hitting enter
-merge_request: 17343
-author:
-type: fixed

changelogs/unreleased/29067-vrt-no-auth.yml

----
-title: Remove authentication step from visual review tools instructions
-merge_request:
-author:
-type: changed

changelogs/unreleased/29165-confusing-wording-fix.yml

----
-title: Fixes wording on runner admin
-merge_request:
-author:
-type: changed

changelogs/unreleased/29713-graphql-add-issue-weight-sorting.yml

----
-title: Graphql query for issues can now be sorted by weight
-merge_request: 19721
-author:
-type: added

changelogs/unreleased/30016-changing-branch-keep-details.yml

----
-title: Keep details in MR when changing target branch
-merge_request: 19138
-author:
-type: changed

changelogs/unreleased/30036-award-emoji-tooltip-is-escaped-twice.yml

----
-title: Fix award emoji tooltip being escaped twice if multiple people voted
-merge_request: 19273
-author: Brian T
-type: fixed

changelogs/unreleased/30390-cablett-duplicate-labels-nonunique-index.yml

----
-title: Add nonunique indexes to Labels
-merge_request: 21230
-author:
-type: fixed

changelogs/unreleased/30666-fix-sr-term-style.yml

----
-title: Changes to how the search term is styled in the results
-merge_request: 20416
-author:
-type: changed

changelogs/unreleased/31031-gitaly-n-1-queries-in-api-version-groups-id.yml

----
-title: Limit number of projects displayed in GET /groups/:id API
-merge_request: 20023
-author:
-type: deprecated

changelogs/unreleased/31236-auto-update-merge-widget.yml

----
-title: Allow merge without refresh when new commits are pushed
-merge_request: 19725
-author:
-type: changed

changelogs/unreleased/31391-update-sparkline-chart-deployment-widget.yml

----
-title: Improve sparkline chart in MR widget deployment
-merge_request: 20085
-author:
-type: other

changelogs/unreleased/31397-go-get-meta-source.yml

----
-title: Support go-source meta tag for godoc.org
-merge_request: 19888
-author: Ethan Reesor (@firelizzard)
-type: changed

changelogs/unreleased/31611-limit-the-number-of-stored-sessions-per-user.yml

----
-title: Resolve Limit the number of stored sessions per user
-merge_request: 19325
-author:
-type: added

changelogs/unreleased/31649-environment-rollback-was-not-friendly.yml

----
-title: Resolve environment rollback was not friendly
-merge_request: 20121
-author:
-type: other

changelogs/unreleased/31662-fix-excessive-find-file-escaping.yml

----
-title: Fix project file finder url encoding file path separators
-merge_request: 21861
-author:
-type: fixed

changelogs/unreleased/31759-clear-cluster-cache.yml

----
-title: Add option to delete cached Kubernetes namespaces
-merge_request: 20411
-author:
-type: added

changelogs/unreleased/31830-limit-mr-target-branches.yml

----
-title: Require group_id or project_id for MR target branch autocomplete action
-merge_request: 20933
-author:
-type: performance