CI: Automate closing and publishing of SonaType repo

[guusdk]

The current GitHub workflows will, upon creating a GitHub release of this project:

1. Create a 'repository' for the release to be created in Sonatype's Nexus Repository Manager (OSS Repository Hosting).
2. Create all artifacts required for publication to the Maven Central repository
3. Use GPG to sign those artifacts
4. Upload them to the repository.

Two steps currently need to be executed manually:

1. Closing the repository (which will start a verification process)
2. Publishing the data in the repository (which makes the artifacts available through the Maven Central repository.

These steps are performed with just a few button clicks in the Nexus repo manager, but it'd be nice if they could be automated.