Update Go compiler to 1.20

Closes #65

See merge request https://gitlab.com/gitlab-org/gitlab-omnibus-builder/-/merge_requests/296

Merged-by: Balasankar 'Balu' C <balasankar@gitlab.com>
Approved-by: Balasankar 'Balu' C <balasankar@gitlab.com>
Co-authored-by: Robert Marshall <rmarshall@gitlab.com>