maintner/maintnerd/maintapi: add supported Go releases to subrepo trybots
maintner/maintnerd/maintapi: add supported Go releases to subrepo trybots

This change computes the supported Go release branches,
and adds them to the GerritTryWorkItem for trybots that
run on subrepos.

This should help significantly with trybots catching regressions
in subrepos that affect older, but supported Go releases.

Fixes golang/go#17626

Change-Id: I4d88f6510c34d0c28266d44c341d5831a8514fce
Reviewed-on: https://go-review.googlesource.com/c/147198
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build