Change image to golang:1.20

What does this MR do?

Follow-up to !166 (diffs, comment 1357247970). The reason why golang:1.17 still worked there was the Deprecation of 'go get' for installing executables coming into effect in v1.18.

This MR bumps Go to v1.20 and adjusts the CI scripts accordingly.

What are the relevant issue numbers?

None directly, but:

• https://gitlab.com/gitlab-org/gitlab/-/issues/287616
• !142 (merged)

Does this MR meet the acceptance criteria?

• [-] Changelog entry added
• [-] Documentation created/updated for GitLab EE, if necessary
• [-] Documentation created/updated for this project, if necessary
• [-] Documentation reviewed by technical writer or follow-up review issue created
• [-] Tests added for this feature/bug
• [-] Job definition updated, if necessary
  • [-] Auto-DevOps template
  • [-] Job definition example
  • [-] CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer