Date block: Update block description

[ockham]

What?

Update the Date block's description.

Why?

Per #70585, the Date block defaults to allowing the user to setting a custom date. (The previous default behavior — showing the publish date of the containing post — was turned into a block variation).

However, the block description was not updated accordingly, as pointed out by @ntsekouras. This PR remediates that.

[ockham]

Note that the "Post Date" and "Modified Date" variations of the block already have more accurate descriptions:

gutenberg/packages/block-library/src/post-date/variations.js

Line 11 in 4d30918

description: __( "Display a post's publish date." ),

gutenberg/packages/block-library/src/post-date/variations.js

Line 33 in 4d30918

description: __( "Display a post's last updated date." ),

[github-actions]

Flaky tests detected in b734ce0.
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/18711485431
📝 Reported issues:

• [Flaky Test] ensures List View global shortcut works properly #51533 in /test/e2e/specs/editor/various/list-view.spec.js

[cbravobernal]

LGTM

[github-actions]

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Co-authored-by: ockham <bernhard-reiter@git.wordpress.org>
Co-authored-by: cbravobernal <cbravobernal@git.wordpress.org>

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[github-actions]

I just cherry-picked this PR to the wp/6.9 branch to get it included in the next release: be5d5fd

[cbravobernal]

Code is already on release/22.0 branch, no need to cherry-pick it.