Add PHP 8.1 to CI pipeline

[Ocramius]

See

ProxyManager/.github/workflows/phpunit.yml

Lines 19 to 20 in 95f9ccf

	php-version:
	- "8.0"

(and similar files in the same directory)

We need to run everything also against PHP 8.1

[nick4fake]

What about php 8.2? It has been already released last year, and this library only supports PHP 8.0