golang: v↑ go1.15 (1.15.10 -> 1.15.11)

Going Go1.15.10 -> Go1.15.11 brings in fixes to compiler, runtime and stdlib:

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).
5 jobs for 1.0 in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.SlapRunner.ResilienceTest-Master

00:26:03

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

00:34:43

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

26:41:04

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

09:00:05

passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master