Update CHANGELOG.md for 8.14.0

[ci skip]

changelogs/unreleased/git-gc-improvements.yml

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-title: Finer-grained Git gargage collection
-merge_request: 6588
-author: 

changelogs/unreleased/hide-empty-merge-request-diffs.yml

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-title: Fix errors happening when source branch of merge request is removed and then restored
-merge_request: 7568
-author:

changelogs/unreleased/issue-13823.yml

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-title: Show random messages when the To Do list is empty
-merge_request: 6818
-author: Josep Llaneras

changelogs/unreleased/issue-24512.yml

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-title: Add placeholder for the example text for custom hex color on label creation popup
-merge_request: 
-author: Luis Alonso Chavez Armendariz

changelogs/unreleased/issue-boards-counter-border-fix.yml

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-title: Fixed issue boards counter border when unauthorized
-merge_request: 
-author: 

changelogs/unreleased/issue_13232.yml

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-title: Add JIRA remotelinks and prevent duplicated closing messages
-merge_request: 
-author: 

changelogs/unreleased/issue_23032.yml

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-title: Allow to test JIRA service settings without having a repository
-merge_request: 
-author: 

changelogs/unreleased/issue_24303.yml

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-title: Fix JIRA references for project snippets
-merge_request: 
-author: 

changelogs/unreleased/issue_5541.yml

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-title: Allow enabling and disabling commit and MR events for JIRA
-merge_request:
-author:

changelogs/unreleased/jira_service_simplify.yml

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-title: simplify url generation
-merge_request:
-author: Jarka Kadlecova

changelogs/unreleased/ldap_check_bind.yml

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-title: Introduce better credential and error checking to `rake gitlab:ldap:check`
-merge_request: 6601
-author: 

changelogs/unreleased/less-intrusive-system-note.yml

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-title: Make system notes less intrusive
-merge_request: 6755
-author:

changelogs/unreleased/mailroom_idle_timeout.yml

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-title: Allow mail_room idle_timeout option to be configurable
-merge_request: 7423
-author:

changelogs/unreleased/master-recursiveTree.yml

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-title: 'API: allow recursive tree request'
-merge_request: 6088
-author: Rebeca Mendez

changelogs/unreleased/milestone-project-require.yml

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-title: Require projects before creating milestone.
-merge_request: 7301
-author: gfyoung

changelogs/unreleased/namespace-validation.yml

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-title: Check all namespaces on validation of new username.
-merge_request: 7537
-author: 

changelogs/unreleased/new-note-worker-record-not-found-fix.yml

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-title: Fix record not found error on NewNoteWorker processing
-merge_request: 6863
-author: Oswaldo Ferreira

changelogs/unreleased/optimize-mr-index.yml

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-title: More aggressively preload on merge request and issue index pages
-merge_request: 
-author: 

changelogs/unreleased/pass-correct-tag-target-to-post-receive.yml

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-title: Pass correct tag target to post-receive hook when creating tag via UI
-merge_request: 7556
-author:

changelogs/unreleased/pipeline-notifications.yml

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-title: Add CI notifications. Who triggered a pipeline would receive an email after
-  the pipeline is succeeded or failed. Users could also update notification settings
-  accordingly
-merge_request: 6342
-author: