go: Use Go 1.19 as minimum required version
We have globally migrated to build all GitLab with Go 1.19, and many of the components have already upgraded to Go 1.19 as their minimum supported version. Let's follow suit and do the same by dropping support for Go 1.18 and starting to test with Go 1.20.
Closes #5324 (closed).
Edited by Patrick Steinhardt