Relocate changelog file for WP#6910 to 6.9 backports

[Mamaduka]

What?

Relocates the backport notes for grid features in #57478.

Why?

The feature will not land in WordPress 6.8. See WordPress/wordpress-develop#6910 (comment).

This is to avoid confusion when merging editor features into WP-Dev.

How?

Moves the file.

Testing Instructions

N/A

Testing Instructions for Keyboard

N/A

Screenshots or screencast

N/A

[github-actions]

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[github-actions]

Flaky tests detected in caa8ea4.
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/13172497022
📝 Reported issues:

• [Flaky Test] should show correct homepage actions based on current homepage or posts page #68892 in /test/e2e/specs/site-editor/homepage-settings.spec.js