Remove deprecations in preparations for Symfony 6

[tacman]

I've started to work on fixing deprecation warnings so that this excellent app can run under Symfony 6 when it comes out in a few weeks.

Symfony 6 requires PHP8, so I've added that to the CI tests.

I'll make a PR soon.

What do you think of having the tests run against Sqlite instead of mysql? It makes local development a bit easier to not require a MySql server.

I've bumped doctrine, and am failing on what is probably something simple, I'll submit a PR and maybe someone can help with whatever I'm doing wrong in the setup.

[j0k3r]

Thanks for taking the time to update the project.
I think it's still important to keep MySQL, it doesn't add too much overload during tests. You can still use SQLite locally it you prefer.

[tacman]

What is the status for Symfony 6 (or now, Symfony 7)