Update CHANGELOG-EE.md for 13.9.0-ee

[ci skip]

ee/changelogs/unreleased/reduce-billing-card-header.yml

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-title: Fix line spacing in billing card header
-merge_request: 52521
-author:
-type: fixed

ee/changelogs/unreleased/remove-enforce_pat_expiration-ff.yml

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-title: Optional enforcement of PAT expiration (feature flag removed)
-merge_request: 53660
-author:
-type: added

ee/changelogs/unreleased/remove_epic_count.yml

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-title: Speed up roadmap show page by removing an extra epic count check
-merge_request: 51610
-author:
-type: performance

ee/changelogs/unreleased/set-merge-option-states.yml

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-title: Enable Merge Train checkbox based on Merge Pipelines checkbox
-merge_request: 51233
-author:
-type: changed

ee/changelogs/unreleased/shaheed121-audit_event_for_default_branch_change.yml

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-title: Adding audit event for default branch change
-merge_request: 52339
-author: Abdul Shaheed
-type: other

ee/changelogs/unreleased/ss-add-iteration-to-bulk-edit-issues.yml

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-title: Add iteration to bulk issue edit sidebar
-merge_request: 51657
-author:
-type: changed

ee/changelogs/unreleased/token_revoke_public_project.yml

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-title: Allow token revocation for public projects only
-merge_request: 53734
-author:
-type: changed

ee/changelogs/unreleased/update-dast-modal-action-button.yml

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-title: Updated text and styling of the DAST-modal footer button
-merge_request: 52245
-author:
-type: changed

ee/changelogs/unreleased/use-parse-boolean-for-iteration-can-edit-value.yml

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-title: Fix invalid prop error for canEdit prop in issue sidebar when user cannot edit iteration
-merge_request: 53819
-author:
-type: fixed

ee/changelogs/unreleased/vij-add-last-activity.yml

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-title: Add last_activity_on attribute to Billable Members API
-merge_request: 52666
-author:
-type: changed

ee/changelogs/unreleased/vij-fix-minutes-cta-namespace.yml

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-title: Fix buy more minutes link to wrong destination
-merge_request: 54080
-author:
-type: fixed

ee/changelogs/unreleased/vij-namespace-limits.yml

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-title: Fix sub-group projects being locked after storage purchase
-merge_request: 53249
-author:
-type: fixed

ee/changelogs/unreleased/vs-fix-faq-link-on-billing-page.yml

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-title: Change FAQ link on the billing page
-merge_request: 52577
-author:
-type: fixed

ee/changelogs/unreleased/vs-update-doc-links-in-usage-quotas.yml

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-title: Replace text with links on Usage Quota page
-merge_request: 53279
-author:
-type: added