Switch CI Nexus backend from OrientDB to H2 and upgrade Nexus to 3.72.0

Following the warning in https://nexus.platform.dev.nuxeo.com/:

Nexus Repository no longer supports OrientDB, Java 8, or Java 11 in future releases. OrientDB users must migrate to H2 or PostgreSQL before upgrading to version 3.71.0. Review our upgrade instructions for versions 3.71.0 and beyond for full details.

We cannot migrate easily to PostgreSQL as it requires a Nexus Pro license as mentioned in https://help.sonatype.com/en/orient-pre-3-70-java-8-or-11.html:

Note that using PostgreSQL requires a Nexus Repository Pro license.

The current version of Nexus is 3.69.0-02 and we are migrating from OrientDB to H2, so we'll have to follow https://help.sonatype.com/en/orient-pre-3-70-java-8-or-11.html.