Upgrade maven to 3.6.3 (the most recent)

Maven needs updating. I pulled in a quarkus dependency which failed with `Error injecting: io.quarkus.maven.BuildMojo` (see, for example, pr/303).

I believe quarkus are on maven version 3.6.2 so I am choosing the latest (which is 3.6.3).

Note that narayana master is on 3.6.1 so that will need bringing into line with this change at some point.