feat: drop support of php < 8, drop support of symfony < 5.4

[Chris53897]

add support symfony 6 https://github.com/kbond/ZenstruckBackupBundle/issues/14

See https://github.com/kbond/php-backup/pull/16

My tests rely on the included test. I did not test it in a real life application.

[kbond]

Thanks for working on this, https://github.com/kbond/php-backup/pull/16, and https://github.com/kbond/ZenstruckRedirectBundle/pull/35 @Chris53897!

The test suites of these three packages need some updating (and switched to github actions). I'll hold off merging until updated and/or test in my app.

[kbond]

Thanks @Chris53897!