Java versions

[gavinbaumanis]

Hey @gareth

Would you consider adding branches for java versions - in place of just having "master"
Eg java-11, java-xx

I have created a java-11 and java-21 branch.
Ensuring each has up to date deps / unneeded deps are removed and also added in some new features.
(same-same for the cflint project, please?)

[ghedwards]

I think I can do better, get the base line back to 11 and then get the builds / ci to do both 21 and 11, will see how I go