These builds are no longer working because some of the dependencies now
require newer versions of Go. Seems like the ecosystem has moved to Go
1.11+, so we are now forced to follow suit.
(cherry picked from commit df7084d36a)
This change adds:
* `tag.yml`: Creates a new tag every time the master or a release branch
is pushed.
* `backport.yml`: Creates a cherry-pick in older release branches to
keep them up to date with little cost.
(cherry picked from commit 2ac9f4e69b)