Add GitHub actions for forward version checking

[pieterocp]

As suggested a while back / in issue form #64

Adds in a couple of GitHub actions to

• run tests over php + supported symfony versions (note, that will require Symfony 8 support #63 to be installable without ignores)
• run tests over the beta version once a month, so that if there's an issue that'll be a breaking change, it can be mitigated before it goes live so this can be readier to go.

(also add in a small change to the phpunit config so that it should in editor know what rules are valid + links to the used version's docs)

(tested this locally + inside my fork)

[Nyholm]

Thank you