FontSizePicker: Hard deprecate 40px default size

[mirka]

What?

Follow up to #65751

Remove the deprecated __next40pxDefaultSize and size props from FontSizePicker, making the 40px default size the permanent default.

Why?

The soft deprecation was introduced in WP 6.7 and scheduled to be removed in WP 7.1.

How?

• Remove __next40pxDefaultSize and size from the FontSizePicker public API and remove maybeWarnDeprecated36pxSize.
• Keep passing __next40pxDefaultSize internally to embedded UnitControl, RangeControl, CustomSelectControl, and ToggleGroupControl until those primitives are hard-deprecated.
• Remove all monorepo usages of the props on FontSizePicker.
• Remove FontSizePicker from the components-no-missing-40px-size-prop ESLint rule and add a restricted-syntax rule for passing the prop to FontSizePicker.

Testing Instructions

1. Open Storybook to Components / FontSizePicker and smoke test the default story.
2. Try preset sizes, the select control (>5 sizes), and custom size mode with slider/reset.
3. Confirm inputs render at 40px height without the prop, and no console warning should be logged.

[github-actions]

Size Change: -138 B (0%)

Total Size: 7.51 MB

📦 View Changed

Filename	Size	Change
build/scripts/block-editor/index.min.js	382 kB	-7 B (0%)
build/scripts/components/index.min.js	264 kB	-131 B (-0.05%)

_{compressed-size-action}

[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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[ciampo]

Red is good 🧹 🚀

[github-actions]

Flaky tests detected in 7dee2e5.
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/28154189165
📝 Reported issues:

• [Flaky Test] should close the inserter when clicking on the toggle button #79248 in /test/e2e/specs/editor/various/inserting-blocks.spec.js