Version 4 - gradle vs maven

[planetf1]

We still have various references relating to maven in our project docs.

Whilst we develop v4 the primary audience is developers, who are exposed to info about gradle via slack, discussions, calls.
Once v4 is released, there's an increasing likelihood of new developers, both using APIs and contributing, to need to use gradle.

I propose to remove our remaining references to maven builds (not maven repos - those remain) around the time we ship release 4.

See odpi/egeria#7330

[planetf1]

Still many references remaining..

[planetf1]

Reviewed, the only remaining references now seem intentional, for older code, or other repos still using maven. Final updates are in #770