Remove changelogs for already-released security patches

changelogs/unreleased/fix-gh-namespace-issue.yml

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-title: Fix namespace access issue for GitHub, BitBucket, and GitLab.com project importers
-merge_request:
-author:
-type: security

changelogs/unreleased/fix-stored-xss-in-code-blocks.yml

----
-title: Fix stored XSS in code blocks that ignore highlighting
-merge_request:
-author:
-type: security

changelogs/unreleased/mc-bug-38984-wildcard-protected-tags.yml

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-title: Fix wilcard protected tags protecting all branches
-merge_request:
-author:
-type: security

changelogs/unreleased/security-10-4-todo-api-reveals-sensitive-information.yml

----
-title: Restrict Todo API mark_as_done endpoint to the user's todos only
-merge_request:
-author:
-type: security