* Update composer requirement to 2.1 PHP 8.1 is coming soon, and it makes Composer 2.0 spit out tons of deprecation warnings. Plus 2.1 has been out for a while now. Seems like a good time to finally update. * CI: Run tests with PHP 8.1, experimentally PHP 8.1 is getting close to release. Let's run tests with it so we can start cleaning stuff up. This is certainly going to fail a lot of stuff for the moment, as WordPress itself is [still working on this][1]. But we may as well get a head start on it. [1]: https://core.trac.wordpress.org/ticket/53635 * Set `convertDeprecationsToExceptions` true in PHPUnit configs PHPUnit is defaulting this to false starting in 8.5.21 and 9.5.10, and recommending everyone change their configs to set it back to true. * Run non-plugin tests when experimental Committed via a GitHub action: https://github.com/Automattic/jetpack/actions/runs/1387011333
54f5724