update generateAntora task to make prereleases unique

[mojavelinux]

Update the generateAntora task in docs subproject to keep the prerelease segment other than -SNAPSHOT (e.g., -RC2) in the docs version. In other words:

name: ROOT
version: 6.0.0-RC2
prerelease: true

and

name: ROOT
version: 6.0.0
prerelease: '-SNAPSHOT'

Only the snapshot should claim the GA version.

[mojavelinux]

This change will need to be propagated to each maintenance branch (5.6, 5.7, 5.8).

[marcusdacoregio]

Hi @mojavelinux, can you change the target branch to 5.6.x? We started to forward-port fixes from the oldest supported branch where the problem happens, which in this case is 5.6.x.

[mojavelinux]

Sure thing. Stay tuned.

[mojavelinux]

Done.

[marcusdacoregio]

Thanks @mojavelinux, this is now merged into all supported branches

[mojavelinux]

Thanks! And onward we go!