golang: Switch default to Go1.18

Go1.18 is the next stable go release. There is already Go1.19
(released 2022-08-02), but we incrementally increase versions.
For Go1.18 there are already many bug fix releases (last one
2022-12-06) which gives it a more stable/mature impression.

/reviewed-on !1305
10 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed Cloudooo.UnitTest-Master

00:08:26

failed ERP5.StandaloneDeploymentScriptTest-Debian.Buster

15:01:50

passed ERP5.StandaloneDeploymentScriptTest-Debian.Buster.Master

07:10:17

passed SlapOS-Master.StandaloneDeploymentScriptTest-Debian.Buster

00:30:49

passed SlapOS.Eggs.UnitTest-Master.Python2

00:15:50

passed SlapOS.Eggs.UnitTest-Master.Python3

00:15:52

passed SlapOS.KVM.ResilienceTest-Master

00:21:14

passed SlapOS.SlapRunner.ResilienceTest-Master

00:23:03

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

00:31:57

failed SlapOS.SoftwareReleases.IntegrationTest-Master

06:26:03