golang: v↑ go1.16 (1.16.3 -> 1.16.4)

Going Go1.16.3 -> Go1.16.4 brings in fixes to compiler, runtime and stdlib
including security fix to net/http package:

https://golang.org/doc/devel/release.html#go1.16.minor

Tested on helloworld SR.
3 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.SlapRunner.ResilienceTest-Master

00:22:48

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

00:38:05

passed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

03:50:25