golang: v↑ go1.15 (1.15.11 -> 1.15.12)

Going Go1.15.11 -> Go1.15.12 brings in fixes to compiler, runtime and stdlib
including security fix to net/http package:

https://golang.org/doc/devel/release.html#go1.15.minor

Tested on helloworld SR (by adjusting it to use go1.15 instead of go1.16).