Add backport changelog entry for #75878

[t-hamano]

What?

Add a backport changelog entry to link #75878 with the core PR.

Testing Instructions

N/A

[github-actions]

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Co-authored-by: t-hamano <wildworks@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>

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[github-actions]

Flaky tests detected in 2e76d82.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/22989560815
📝 Reported issues:

• [Flaky Test] should undo bold #48558 in /test/e2e/specs/editor/various/undo.spec.js