feat: change default version to Go 1.20

What

1. Run tests for 1.18 - 1.20
2. Run backward compatibility checks on go 1.19
3. Remove -compact flag

Why

1. We need to keep go 1.18 support since GitLab 15.10 is still running 1.18
2. Gitaly updated the minimum version to go 1.19 gitaly!5831 (merged) so backward compatibility jobs are failing
3. -compact flag was only a temporary measure to support go 1.17 in !171 (merged) and no longer needed.

Closes: #61 (closed)