Update PHP to 8.1

[rvalitov]

Currently it's stated that this project works with PHP8+. However, using PHP8 causes error because of the cache dependency #62:

php composer update                                  
Loading composer repositories with package information
Updating dependencies
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Root composer.json requires symfony/cache ^6.3 -> satisfiable by symfony/cache[v6.3.0, ..., v6.4.20].
    - symfony/cache[v6.3.0, ..., v6.4.20] require php >=8.1 -> your php version (8.0.30) does not satisfy that requirement.

This pull request updates the PHP version requirement for the project both in README and composer.json file.
Fixes #89.

[rvalitov]

All tests fail automatically because of outdated action upload-artifact, see #86

[rvalitov]

Superseded by #88