Release automation overhaul: Sonatype Nexus, Shipkit and GH Actions

[mockitoguy]

This change is backwards compatible, please review!

• enables concurrent releases
• avoids unnecessary version bump commits
• migrates off discontinued tech (Bintray, Shipkit 2.x)
• GH Actions are faster and more convenient to use than Travis CI

Implemented migrations:

• Bintray -> Sonatype Nexus
• Travis -> GH Actions (kept simple travis build for comparing)
• Shipkit 2.0 -> Shipkit 3.0 (new plugins)

Tested by:

• ./gradlew build - builds, runs tests
• ./gradlew publish - publishes to rootDir/build/repo, artifacts look good
• ./gradlew githubRelease - release correctly published to https://github.com/mockitoguy/coral/releases/tag/v1.0.28
• ./gradlew publishToSonatype - archives correctly published to staging repo on Sonatype

Post-merge actions:

For releases from CI to work, maintainers need to export GitHub secrets so that they are available in GH Actions:

1. For publishing to Sonatype Nexus / Maven Central configure SONATYPE_USER and SONATYPE_PWD secrets. Official guide on publishing to Sonatype Nexus: https://central.sonatype.org/pages/ossrh-guide.html
2. For signing, we need PGP_KEY and PGP_PWD secrets. Here's a somewhat useful guide: https://medium.com/@nmauti/sign-and-publish-on-maven-central-a-project-with-the-new-maven-publish-gradle-plugin-22a72a4bfd4b Hint: once you generate the PGP key, here's how you can view it: gpg --export-secret-keys -a KEY_ID

[mockitoguy]

@wmoustafa, merging this needs some work on your end (exporting secrets to GH Actions, verifying e2e releases with Maven Central). I added details to PR description and I hope they are useful!

[wmoustafa]

Thanks @mockitoguy! This looks good. I think it is similar to the approach in #51, but uses the new Shipkit plugins and Github Actions. I have setup the secrets.