Update CHANGELOG.md for 12.8.0

[ci skip]

changelogs/unreleased/weimeng-upgrade-omniauth-github.yml

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-title: Upgrade omniauth-github gem to fix GitHub API deprecation notice
-merge_request: 24928
-author:
-type: fixed

changelogs/unreleased/xanf-prohibit-outer-forks.yml

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-title: Added "Prohibit outer fork" setting for Group SAML
-merge_request: 25246
-author:
-type: added

changelogs/unreleased/youtrack-issue-pattern.yml

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-title: Avoid autolinking YouTrack issue numbers followed by letters
-merge_request: 24770
-author: Konrad Borowski
-type: fixed