Migrate travis to github workflows?

[lsf37]

Now that github workflows are free for public repos, it is probably easier to maintain and easier to interact with if we run the travis CI tasks directly on github.

@regisd and @sarowe, unless you see any concerns with that, I might give this a go when there is a bit of time and put up a test pull request.

[regisd]

For Maven, it makes perfect sense.

Bazel, CIrrus-CI supports remote cache, and I think Github doesn't.

[lsf37]

Happy to leave Bazel on Cirrus, it seems better integrated into Github that Travis in any case.

[lsf37]

#768 contains enough progress for this to replace travis as the required check for the regression test suite, but there are a few things still to be set up:

• [x] set up build badge in README, remove travis build badge
• [x] matrix build for multiple java versions (todo)
• [x] possibly a build in the matrix for MacOS (either todo or decide not to do)
• [x] ~~send test coverage results (needs auth token)~~
• [x] ~~deploy github pages~~
• [x] replace the update of aggregated-java-sources by Github

After that, we can

• [x] remove travis build

[lsf37]

https://github.com/jflex-de/jflex/pull/1005 has now moved the Bazel build to GitHub actions as well. The cache is implemented locally with the GH cache action.

[lsf37]

Deploying the jflex maven plugin site doesn't make that much sense, because it would deploy the snapshot version, not the release version. We had already removed this from Travis quite a while ago, i.e. this doesn't have much to do with migrating from Travis/Cirrus to GitHub actions. Leaving this on manual for now (it's covered in the release docs).