Migrate to GH actions

[mockitoguy]

Migrating to GH actions improves the CI/CD experience (the UI is cleaner, it's easier to analyze logs, the jobs start quicker).

Mockito (mockito/mockito#2080) and mockito-testng (mockito/mockito-testng#17) projects have already migrated to GH Actions.

[ultrasecreth]

Nice, I'll have a look this weekend I hope