Move to GitHub actions for all build related steps

[MasterOdin]

With #1759, GitHub actions, Travis, and appveyor are all used to build and test phinx in different aspects. Given that GitHub actions supports Windows and any functionality of Travis (beyond debugging builds, which it does not seem like phinx uses), it feels like it would make sense to move everything to one build environment, and phase out the others, making it also easier to just maintain build environments going forward.

I believe looking at cakephp/cakephp that a reason to keep Travis and GH actions was for codecov, but it seems like https://github.com/codecov/codecov-action would indicate that is not necessary.

Assuming there's buy-in from the cakephp folks, I'd be happy to take this on (as I move everything else I work on to GH actions over Travis these days).

[othercorey]

I am going to update the CI builds soon. We'll keep in sync with cakephp core.