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Don't drop Go1.12 and Go1.13 yet: they are too outdated and no longer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.

component/golang/buildout.cfg

@@ -32,33 +32,6 @@ environment-extra =
 make-targets= cd src && ./make.bash && cp -alf .. ${:location}
 
 
-[golang19]
-<= golang-common
-url = https://dl.google.com/go/go1.9.7.src.tar.gz
-md5sum = 3c2cf876ed6612a022574a565206c6ea
-
-# go1.9 needs go1.4 to bootstrap
-environment-extra =
-  GOROOT_BOOTSTRAP=${golang14:location}
-
-[golang1.10]
-<= golang-common
-url = https://dl.google.com/go/go1.10.3.src.tar.gz
-md5sum = d15dfb264105c5e84fbe33f4a4aa5021
-
-# go1.10 needs go1.4 to bootstrap
-environment-extra =
-  GOROOT_BOOTSTRAP=${golang14:location}
-
-[golang1.11]
-<= golang-common
-url = https://dl.google.com/go/go1.11.4.src.tar.gz
-md5sum = a77697673215be465d1b583680ef2318
-
-# go1.11 needs go1.4 to bootstrap
-environment-extra =
-  GOROOT_BOOTSTRAP=${golang14:location}
-
 [golang1.12]
 <= golang-common
 url = https://golang.org/dl/go1.12.17.src.tar.gz