Remove already released CHANGELOG files

changelogs/unreleased/38126-security-username-change.yml

----
-title: Move project repositories between namespaces when renaming users
-merge_request:
-author:
-type: security

changelogs/unreleased/fix-arbitrary-redirect-vulnerability.yml

----
-title: Prevent an open redirect on project pages
-merge_request:
-author:
-type: security

changelogs/unreleased/rs-sanitize-unicode-in-protocol.yml

----
-title: Prevent a persistent XSS in user-provided markup
-merge_request:
-author:
-type: security