General: compatibility with PHP 8.4

[jeherve]

The Core WordPress team is still working on PHP 8.4 compatibility here. While there are still things to change in Core to ensure full compatibility, we can also work on compatibility within the Jetpack plugins.

For a better view of WordPress's PHP compatibility and support, you can refer to this page.

Tasks

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1. Replace E_USER_ERROR for PHP 8.4 #39187
   [Package] Stub Generator [Plugin] Jetpack [Pri] Normal [Tests] Includes Tests [Type] Janitorial +5
   
2. CI: Disable 8.4 PHPCompatibility rule in dev run #36673
   Actions [Package] Identity Crisis [Package] My Jetpack [Plugin] Boost [Pri] BLOCKER [Type] Enhancement +6
   
3. ci: Test against PHP 8.4
4. codesniffer: add Jetpack-Compat 8.4 ruleset

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For reference: PHP 8.2 compatibility was tracked in #37173