migrate CI build to github actions

[bradh]

As requested on the mailing list, migrates most of the Travis CI build activity to Github Actions.

Checklist

For all pull requests:

• Confirm you have read the contribution guidelines
• You have sent a Contribution Licence Agreement (CLA) as necessary (not required for small changes, e.g., fixing typos in documentation)
• Make sure the first PR targets the master branch, eventual backports will be managed later. This can be ignored if the PR is fixing an issue that only happens in a specific branch, but not in newer ones.

The following are required only for core and extension modules (they are welcomed, but not required, for community modules):

• [N/A] There is a ticket in Jira describing the issue/improvement/feature (a notable exemptions is, changes not visible to end users)
• [N/A] PR for bug fixes and small new features are presented as a single commit
• [N/A] Commit message must be in the form "[GEOS-XYZW] Title of the Jira ticket" (export to XML in Jira generates the message in this exact form)
• The pull request contains changes related to a single objective. If multiple focuses cannot be avoided, each one is in its own commit and has a separate ticket describing it.
• [N/A] New unit tests have been added covering the changes
• This PR passes all existing unit tests (test results will be reported by travis-ci after opening this PR)
• This PR passes the QA checks (QA checks results will be reported by travis-ci after opening this PR)
• [N/A] Commits changing the UI, existing user workflows, or adding new functionality, need to include documentation updates (screenshots, text)
• [N/A] Commits changing the REST API, or any configuration object, should check if the REST API docs (Swagger YAML files and classic documentation) need to be updated.

[aaime]

This is great progress! The missing bit here seems to be the packaging build (ensuring we can build the war/bin and the extension zip packages without error).

[bradh]

This is great progress! The missing bit here seems to be the packaging build (ensuring we can build the war/bin and the extension zip packages without error).

I think this is done. I've folded the package steps into the oraclejdk11 job (see https://github.com/geoserver/geoserver/pull/4581/checks?check_run_id=1458114425), with a (perhaps optimistic) hope that we can reuse the bits we just built, before removing the -SNAPSHOT jars from the cache.

[bradh]

Travis is failing with a ConcurrentModificationException, which looks unrelated. Have kicked that build, but don't think it will be relevant.

[aaime]

Oh... do you remember where it was? I've recently changed some code related to concurrency (to get higher scalability) and worried that failure might be related.

[bradh]

Oh... do you remember where it was? I've recently changed some code related to concurrency (to get higher scalability) and worried that failure might be related.

Security core, maybe.

[mprins]

👍

[aaime]

Squash-merged for ease of backport. Thanks for the hard work, much appreciated!

[bradh]

Added backports to 2.17.x and 2.18.x