This included a couple of breaking changes:
- The build plugin is now part of the Gradle Enterprise plugin
- The osgi plugin was removed, for which we have to use the bndtools
plugin instead
- Kotlin version has to be updated, including their coroutine
library versions

Gradle 6 is required for us to build on Java 14.

Supersedes and closes #1923

Gradle 6 does not like that we run a build twice, as part of a Gradle
task. Therefore, we have to move the checksum computation out of
the gradle task in a separate Bash script.

Otherwise we lose the code coverage, as we run a `gradle clean`
in the reproducibility check.

This now also makes use of the build cache, which should significantly
speed up Travis builds.