Real-time collaboration: Prevent fall-through in sync polling server

[chriszarate]

Trac ticket: https://core.trac.wordpress.org/ticket/64781

When a user sends a stale compaction update to WP_HTTP_Polling_Sync_Server, it falls through the switch statement and results in a rest_invalid_update_type error (bad request).

This generally has no ill effect since the compaction request is rescinded on the next polling cycle, but it could cause confusion for users who are monitoring client-side requests or server logs.

To fix, replace the break after the $has_newer_compaction check with return true so that the stale compaction is silently discarded instead of falling through. Covered by unit test.

Backport of WordPress/gutenberg#76060

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