Update CHANGELOG.md for 12.3.0

[ci skip]

changelogs/unreleased/update-babel-to-7-5-5.yml

----
-title: Upgrade babel to 7.5.5
-merge_request: 31819
-author: Takuya Noguchi
-type: other

changelogs/unreleased/update-gitlab-runner-helm-chart-to-0-8-0.yml

----
-title: Update GitLab Runner Helm Chart to 0.8.0
-merge_request: 32289
-author:
-type: other

changelogs/unreleased/update-workhorse.yml

----
-title: Update GitLab Workhorse to v8.10.0
-merge_request: 32501
-author:
-type: other

changelogs/unreleased/use_default_external_auth_label_empty.yml

----
-title: Prevent empty external authorization classification labels from overriding
-  the default label
-merge_request: 32517
-author: Will Chandler
-type: fixed

changelogs/unreleased/user_name_migration.yml

----
-title: Add First and Last name columns to User model
-merge_request: 31985
-author:
-type: added

changelogs/unreleased/winh-deduplicate-board-headers.yml

----
-title: Hide duplicate board list while dragging
-merge_request: 32099
-author:
-type: fixed