Improve behaviour of maven semantic versioning

The maven semantic versioning in maven.gitlab-ci.yml has a couple of bugs:

• it doesn't currently fully/correctly implement conventional commits
• additionally it can be seen to create versions which contain -SNAPSHOT-SNAPSHOT as the suffix, as seen in https://gitlab.developers.cam.ac.uk/uis/devops/hr/wr/wrs-webapp/-/jobs/2194097. This may be an issue due to a manually pushed build being present within the GitLab package registry, in which case we are limited in terms of how the script can account for this. We should provide some docs regarding resolving issues due to manually pushed builds if needed. We have also seen this when there hasn't been any manual pushes to the registry.
• calculated next version for some projects is not correctly incrementing in sequence and is updating previously published versions, see this job, and this job that replaced an existing 1.1.1 version that was from the previous merge to main

I (@sw2187) have implemented some tests to express what I think (but to be discussed) should be some characteristics of how this versioning should behave.