Main goal of this change is to phase out the use of
JCenter before it is [sunset].

[sunset]: https://jfrog.com/blog/into-the-sunset-bintray-jcenter-gocenter-and-chartcenter/

The following updates have been made:

- Move from TravisCI to Github Actions.
- Update to latest Nebula plugins.
- Update to latest Gradle and codequality plugins.
- Fix a few issues flagged by updated codequality plugins.
- Remove API compatibility check. This broke for some
  and given the current state of Servo, it was just
  removed rather than trying to fix.