Make 8.x checks a parameter of the action

[pwtyler]

Instead of using an @dev branch whose only difference is using an unreleased version of the upstream, make the option for 8.x checks an input parameter of running the action

[benoitchantre]

I don’t understand why the test-versions defaults to 8.0-, but you need to use the @dev branch to benefit from appropriate dependencies.

@pwtyler What would be the downsides to merge the develop branch to main?

[pwtyler]

I assume @kporras07's intention was to not have the "stable" branch of the action point to an unstable version of the PHPCompatibility project, but since we're at 4+ years since a release, it's likely the right choice to just default to using the develop branch of PHPCompatibility.

[kporras07]

Yeah, also I think I initially didn't notice that you needed dev to get those versions. I'm happy with the decision that @pwtyler makes here :)