Build cache at 'https://ge.spring.io' is not a Develocity build cache that is compatible with Maven builds

The cause is an inconsistency between Develocity's Gradle plugin and its Maven extension. The former uses something like https://ge.spring.io as the URL for the cache and then automatically appends /cache/ to it. The latter requires a URL like https://ge.spring.io/cache/.

We can update the conventions to append /cache/ when we're configuring the Maven side of things.