CI: run the test matrix on PHP 8.1

[roxblnfk]

PHP 8.1 support was added in #5, but the CI matrix still runs only on 8.2+.

The test runner (Testo) does not support PHP 8.1 yet, so the suite cannot be executed on 8.1 in CI. Blocked by:

• PHP 8.1 support php-testo/testo#322

Once Testo supports 8.1, add 8.1 to the CI matrix (tests, Psalm, code style) in a separate PR.