Backport: Disable real-time collaboration when revisions are off

[maxschmeling]

Summary

Backports WordPress/gutenberg#77502 for Gutenberg issue #77499.

PHP

• Introduces wp_revisions_are_globally_supported() and gates wp_is_collaboration_enabled() so RTC stays off when WP_POST_REVISIONS disables revisions site-wide.
• Settings -> Writing: inline notice and disabled Collaboration checkbox when revisions are globally unavailable.

JavaScript

Built bundles live under src/wp-includes/js/ (gitignored). Rely on the usual Core sync workflow before merge so the built editor and preferences assets include the corresponding Gutenberg JavaScript changes.

Tests

Adds PHPUnit coverage for wp_revisions_are_globally_supported() and the enabled gate under default test configuration.

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See the Gutenberg PR description for manual testing steps (WP_POST_REVISIONS, Writing screen, editor preferences).

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