Sunset Maven build descriptor

[msteindorfer]

The Capsule project contains two build descriptors, one for Maven and one for Gradle. The former is out of sync and, e.g., does not cover recent features (cf. #35). In order to avoid feature disparity and the two descriptors getting out of sync, I suggest removing the Maven build descriptors in favor of Gradle.

Steps for sunsetting the Maven build descriptor:

• Update GitHub Actions to use Gradle (i.e., test pipelines and release workflow)
• Migrate JaCoCo to Gradle
• Find a suitable and maintained Gradle equivalent for com.mycila:license-maven-plugin

[jurgenvinju]

Hi Michael; all of our builds are done with maven. We don't have any Gradle experience or expertise in the team. And we manage a number of our own maven plugins. So I would hesitate to switch to only Gradle. Could we do it the other way around?