Move validation test suite, phpstan to GitHub Workflows

[TysonAndre]

• Perform a shallow clone of submodules analyzed in validation
  test suite in GitHub workflows (should take around 30 seconds)
• Switch badge in README.md to GitHub Workflows and update default
  branch for badge.
• Remove travis config
• After PHPStan Level 3 #385 is merged run_phpstan.sh can remove the separate install
  step (as a followup PR)
• Require a newer phpunit patch version in devDependencies
  Make it harder for contributors to have any inconsistencies with CI
  when running tests (e.g. php version support, fixed phpunit bugs, etc)

[TysonAndre]

This is ready for review, no other changes are planned

• phpstan successfully ran in php 7.2 (needed to use matrix.VALIDATION)
• The validation folder is now mounted as a read-only volume when running tests with ci/run_tests_dockerized.sh and the test suite runs on the 10000+ php files found in validation/frameworks
• This was rebased against the latest commits

[roblourens]

Awesome, thank you for doing this!

[roblourens]

I've merged 385 and removed a merge conflict with main, do you want to remove this now?

[roblourens]

This separate step can be removed now though right?

[TysonAndre]

I undid the changes to php versions in github actions to avoid conflicts with https://github.com/microsoft/tolerant-php-parser/pull/406/files#diff-7829468e86c1cc5d5133195b5cb48e1ff6c75e3e9203777f6b2e379d9e4882b3 and updated composer.json again

Unit tests and validation steps aren't run until the workflow run is approved https://docs.github.com/en/actions/managing-workflow-runs/approving-workflow-runs-from-public-forks

[roblourens]

It looks like phpstan ran out of memory

     PHPStan process crashed because it reached configured PHP memory limit:   
     128M      

[dantleech]

It looks like phpstan ran out of memory

if that's the only issue it can be fixed with phpstan --memory-limit=4G or similar cc @TysonAndre