golang: Drop support for Go1.9, Go1.10 and Go1.11

These are old Go releases that were stopped being updated in 2018 - mid
2019:

https://golang.org/doc/devel/release.html#go1.9
https://golang.org/doc/devel/release.html#go1.10
https://golang.org/doc/devel/release.html#go1.11

and that are not used by any in-tree SR or component.

Don't drop Go1.12 and Go1.13 yet: they are too outdated and no longer
supported, but those Go releases are still in use by the following
slapos software:

Go1.12: Caddy, GitLab, Grafana
Go1.13: RepMan, Restic

The maintainers of those software are advised to upgrade to Go1.14 due
to at least recent security fixes in net/http package.