Check PHP 8.1 support

[devanych]

At the moment, tests on PHP 8.1 version are falling with an error:

TypeError: Memcached::set Multi(): Argument #2 ($expiration) must be of type int, int given

And:

Failed asserting that -1.1 matches expected 1.1.

See:

php-memcached-dev/php-memcached#496

https://github.com/yiisoft/cache-memcached/runs/4954577582?check_suite_focus=true