Check environment.xml for DB minimums and ensure services are registered accordingly

[keevan]

I'm happy to get this over the line so the tests can resume as normal, but ideally it would use a higher version of:

• the test matrix
• or sniffing it out via the environment.xml or similar to deduce the minimum DB version requirements for the branch.

Allowing this to be dynamic would be a nice to have, and might reduce the need to update this periodically. But that can be pushed down the road if we can get this working again relatively easily.

I'll split this into another enhancement issue.

Originally posted by @keevan in #56 (comment)

• Deduce the minimum from the environment.xml (or similar) of the moodle branch, and supply it back when creating the DB services
• Use both the 'includes matrix' and environment to dynamically deduce the most appropriate DB version to use for testing the branch. In future, we'd bump the minimums in the 'includes matrix' if we think a higher versioned DB would perform better, but this isn't a strict requirement.