Upgrade the minimum required Go version to Go 1.18
We had been blocked on upgrading to Go 1.18 for quite some time now as several of GitLab's dependencies didn't yet support this new version. Now that all dependencies in &8843 (closed) have been fixed though we're unblocked on the version upgrade. This MR thus bumps the minimum required Go version to Go 1.18 and addresses any fallout caused by this.