feat: change default go version to 1.20

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

Why
---
1. We need to keep go 1.18 support since GitLab 15.10 is still running
   [1.18](https://gitlab.com/gitlab-org/build/CNG/-/blob/15-11-stable/ci_files/variables.yml?_gl=1%2ax9rmw0%2a_ga%2aMTQzNDkyOTA3Ny4xNjY1OTk4ODU3%2a_ga_ENFH3X7M5Y%2aMTY4NjA1Mjc1OC4yNzguMC4xNjg2MDUyNzU4LjAuMC4w&ref_type=heads#L22)
1. Gitaly updated the minimum version to go 1.19
   gitaly!5831 so
   backward compatibility jobs are failing
1. `-compact` flag was only a temporary measure to support go 1.17 in
   !171 and [no
   longer
   needed](!171 (comment 1149031394)).

Closes: #61


Signed-off-by: Steve Azzopardi <sazzopardi@gitlab.com>