Use GithubActions instead of Travis & AppVeyor

[OskarStark]

Todos

• for PHP-CS-Fixer & composer validate Enhancement: Run PHP-CS-Fixer & composer validate via GithubActions instead of Travis #694
• for tests (Linux) Enhancement: Use GithubActions for tests instead of Travis + PHP 8.0 tests #701
• for tests (Windows) - currently not possible Enhancement: Use GithubActions for tests instead of AppVeyor #714
• fix PHP 8 tests Fix: PHP 8 build #715
• deactivate Travis Webhook - @weaverryan

[OskarStark]

@weaverryan you can deactivate the travis webhook and close this issue.
We keep AppVeyor for now as mentioned in #714

[weaverryan]

Travis webhook deactivated - thanks @OskarStark!

[OskarStark]

🕺