Update CHANGELOG-EE.md for 13.10.0-ee

[ci skip]

ee/changelogs/unreleased/terrichu-fix-notes-to-es-support-null-projects.yml

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-title: Only set note visibility_level if associated to a project
-merge_request: 55111
-author:
-type: fixed

ee/changelogs/unreleased/user-permission-docs.yml

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-title: User permission export (feature flag removed)
-merge_request: 53789
-author:
-type: added

ee/changelogs/unreleased/users-statistics-page-updates.yml

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-title: Correct billable users calculation on users statistics page
-merge_request: 56152
-author:
-type: fixed

ee/changelogs/unreleased/vij-fix-billable-member-removal.yml

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-title: Add remove Billable Member API endpoint
-merge_request: 55645
-author:
-type: added

ee/changelogs/unreleased/vs-add-last-activity-column.yml

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-title: Add last activity time column to seat usage table
-merge_request: 55321
-author:
-type: added

ee/changelogs/unreleased/vs-add-remove-button-to-billable-members.yml

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-title: Add remove button to billable members
-merge_request: 54458
-author:
-type: added

ee/changelogs/unreleased/widgetize-iteration-component-v1.yml

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-title: Add a separator between milestone and iteration dropdown in sidebars
-merge_request: 54895
-author:
-type: other