Remove changelog entries for 8.16.5 release

changelogs/unreleased/asciidocs-xss-patch.yml

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-title: Patch Asciidocs rendering to block XSS
-merge_request:
-author:

changelogs/unreleased/fix-xss-svg.yml

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-title: Fix XSS vulnerability in SVG attachments
-merge_request:
-author:

changelogs/unreleased/labels-assigned-to-wrong-project.yml

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-title: Prevent the GitHub importer from assigning labels and comments to merge requests or issues belonging to other projects.
-merge_request:
-author:

changelogs/unreleased/patch-rdoc-xss.yml

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-title: Patch XSS vulnerability in RDOC support
-merge_request:
-author: