Update CHANGELOG.md for 8.16.0

[ci skip]

changelogs/unreleased/issue_25682.yml

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-title: Parse JIRA issue references even if Issue Tracker is disabled
-merge_request:
-author:

changelogs/unreleased/issues-8081.yml

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-title: change 'gray' color theme name to 'black' to match the actual color
-merge_request: 7908
-author: BM5k

changelogs/unreleased/ldap_maint_task.yml

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-title: Add LDAP Rake task to rename a provider
-merge_request: 2181
-author: 

changelogs/unreleased/login-page-font-size.yml

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-title: Decreases font-size on login page
-merge_request:
-author:

changelogs/unreleased/markdown-area-height-fix.yml

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-title: Autoresize markdown preview
-merge_request: 8607
-author: Didem Acet

changelogs/unreleased/mr-tabs-alignment-sidebar-open.yml

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-title: Fixed merge request tabs dont move when opening collapsed sidebar
-merge_request:
-author:

changelogs/unreleased/nuke-ugly-spaces-in-changelog-generator.yml

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-title: Remove trailing whitespace when generating changelog entry
-merge_request: 7948
-author:

changelogs/unreleased/participants-list.yml

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-title: Fix participants margins to fit on one line
-merge_request:
-author:

changelogs/unreleased/pc-add-gitaly-to-architecture.yml

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-title: Add Gitaly to the architecture documentation
-merge_request: 8264
-author: Pablo Carranza <pablo@gitlab.com>

changelogs/unreleased/pipelines-graph-html-css.yml

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-title: Fixes and Improves CSS and HTML problems in mini pipeline graph and builds dropdown
-merge_request: 8443
-author:

changelogs/unreleased/pmq20-gitlab-ce-psvr-head-cache.yml

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-title: Expire related caches after changing HEAD
-merge_request:
-author: Minqi Pan

changelogs/unreleased/re-style-issue-new-branch.yml

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-title: Remove checking branches state in issue new branch button
-merge_request: 8023

changelogs/unreleased/recaptcha_500.yml

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-title: Properly handle failed reCAPTCHA on user registration
-merge_request: 8403
-author:

changelogs/unreleased/record-used-ssh-keys-once-per-day.yml

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-title: Record used SSH keys only once per day
-merge_request: 8655
-author:

changelogs/unreleased/reduce-queries-milestone-index.yml

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-title: Use cached values to compute total issues count in milestone index pages
-merge_request: 8518
-author:

changelogs/unreleased/refresh-authorizations-tighter-lease.yml

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-title: Synchronize all project authorization refreshing work to prevent race conditions
-merge_request:
-author:

changelogs/unreleased/remove-project-authorizations-id-column.yml

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-title: Remove the project_authorizations.id column
-merge_request:
-author:

changelogs/unreleased/remove-successful-pipeline-emails-for-now.yml

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-title: Make successful pipeline emails off for watchers
-merge_request: 8176
-author:

changelogs/unreleased/restore-backup-when-env-variable-is-passed.yml

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-title: Restore backup correctly when "BACKUP" environment variable is passed
-merge_request: 8477
-author:

changelogs/unreleased/sandish-gitlab-ce-update_ret_val.yml

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-title: Ensure updating project settings shows a flash message on success
-merge_request: 8579
-author: Sandish Chen