Add track "7" to gam-bp

The original gam-bp snap was published when GAM 6.x was still current - the v7 rewrite broke compatibility upstream, so it’s unreasonable to push 7.x releases to latest/stable and break installations for people still using 6.x releases.

I’d like a new 7/ track so that I can push those to e.g. 7/stable, 7/candidate, etc instead, please.

I’d ideally like to also rename the latest/ track to 6/ for clarity too (since 6.x is NOT the latest) - but I think that may leave users tracking a latest/x channel with orphaned installations? Are there any specific recommendations for this kind of situation?

Thanks!