* GH Actions: run validatePullRequest instead of just compiling the tests
* Run MiMa checks on GH Actions instead of Travis CI
* Some clear names for workflow and steps
* Add cache to bin compat step
* Also run MiMa for master branch and tags
* Use JDK 8 to validate pull request
* Move validateCompile to pull request validation
Also remove `update` and `Test/compile` from binary compatibility checks:
- `validateCompile` runs for all configurations, so it overrides `Test/compile`
- `update` is unnecessary for mima checks
