Upgrade to Go 1.19

What does this MR do?

It upgrades the whole project to Go 1.19.

Why was this MR needed?

I want to be able to use this in the new Verify Releaser project as a library and reusing the logic in this project is vital. In the far future the two will be merged but for now this is the sensible approach.

This is also the stepping stone for !32 (merged) which actually exposes library functionality.

What's the best way to test this MR?

Pipeline should pass

What are the relevant issue numbers?

Related to gitlab-org/gitlab-runner#29445 (closed)