Remove already released CHANGELOG entries

changelogs/unreleased-ee/3435-related-issues-system-notes-are-not-confidential.yml

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-title: Prevent Related Issues from leaking confidential issues
-merge_request: 541
-author:
-type: security

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

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

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

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

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

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