Set up build to publish via Maven Central Portal

[marcphilipp]

Overview

Snapshots are now being published to
https://central.sonatype.com/repository/maven-snapshots and the
JReleaser Gradle plugin is configured for publishing releases.

Resolves #4483.

---

I hereby agree to the terms of the JUnit Contributor License Agreement.

---

Definition of Done

• There are no TODOs left in the code
• Method preconditions are checked and documented in the method's Javadoc
• Coding conventions (e.g. for logging) have been followed
• Change is covered by automated tests including corner cases, errors, and exception handling
• Public API has Javadoc and @API annotations
• Change is documented in the User Guide and Release Notes