Disable updating GB, MPFA, M3A and UN XSLT in metanorma gems

[Intelligent2013]

As titled (update Makefile and workflows)

From https://github.com/metanorma/metanorma-un/issues/279:

@Intelligent2013 That means that as with metanorma-gb, metanorma-mpfa and metanorma-m3aawg, I will no longer be maintaining this gem, and you should stop maintaining its associated XSLT as well.

[Intelligent2013]

Done.