Remove Go 1.18 Operator build image
What does this MR do and why?
Remove Go 1.18 Operator build image
The last three Operator minor releases use the Go 1.20 image as the
build image. Therefore, the Go 1.18 image can be removed.
Related to !705 (comment 1499967569)
Checklist
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(If applicable) Add patches if required for upgrading Ruby version in https://gitlab.com/gitlab-org/gitlab-build-images/-/tree/master/patches/ruby.
Edited by Clemens Beck