QueryControls: Complete __next40pxDefaultSize cleanup

[mirka]

What?

Follow up to #65751 and #64457

QueryControls already unconditionally renders 40px child controls. This PR finishes the remaining bookkeeping: docs, internal prop threading, and ESLint.

Why?

The public __next40pxDefaultSize prop was deprecated in WP 6.7 and has been a noop since #64457 defaulted the component to 40px internally. This aligns docs and lint rules with that reality.

How?

• Remove __next40pxDefaultSize from the README.
• Stop threading the prop through internal AuthorSelect and CategorySelect; hardcode it on embedded TreeSelect instances instead.
• Remove the prop from internal AuthorSelectProps and CategorySelectProps types.
• Add a restricted-syntax rule to forbid passing __next40pxDefaultSize to QueryControls.

No runtime behavior change.

Testing Instructions

1. Open Storybook to Components / QueryControls and smoke test the default and single-category stories.
2. Insert a Latest Posts block and confirm the query controls in the sidebar still render at 40px height.
3. Confirm no console warnings are logged.

[github-actions]

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Co-authored-by: mirka <0mirka00@git.wordpress.org>
Co-authored-by: ciampo <mciampini@git.wordpress.org>

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[github-actions]

Size Change: -8 B (0%)

Total Size: 7.51 MB

📦 View Changed

Filename	Size	Change
build/scripts/components/index.min.js	264 kB	-8 B (0%)

_{compressed-size-action}

[github-actions]

Flaky tests detected in 3a46667.
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/28157001491
📝 Reported issues:

• [Flaky Test] Should save the changes #78689 in /test/e2e/specs/editor/plugins/wp-editor-meta-box.spec.js
• [Flaky Test] should be preserved on first navigation with data-wp-key and attachTo #75767 in /test/e2e/specs/interactivity/router-regions.spec.ts

[ciampo]

🧹 🚀