feat: drop support of php 7.x, drop support symfony < 5.4

[Chris53897]

, add support symfony 6, add strong types.

Please check for BC checks of strong types.

I am not sure about these changes. Nullable or not.
https://github.com/kbond/php-backup/compare/master...Chris53897:php-backup:feature/modernize?expand=1#diff-abf1fa77a19f720052b75bc802df0260157b9064d8c3c12a80794f01e9acc065R130

I did not run the 3 tests of mysqldump (as not installed at my system)
The rsync output is a bit different. That may differs from rsync version.

To test final classes this can be used (https://phpunit.readthedocs.io/en/9.5/test-doubles.html).
I did not use it before. Or do you have a better idea?
https://tomasvotruba.com/blog/2019/03/28/how-to-mock-final-classes-in-phpunit/

Improvements:
Flysystem v3 instead of v1.
GitHub Actions instead of Travis (I would suggest to do both in separate PR's).

[kbond]

Thanks @Chris53897!