go-backcompat: create back-compat guidance as annotation

Created by: bobheadxi

Generate migration failure warning as an annotation. This should happen rarely enough to warrant taking up the extra space, and I think should primarily happen on PRs?

(aside: I just want to assign a multi line string to a variable but it kept failing 😞 tried a bunch of stack overflows to make this work ugh bash why)

Test plan

https://buildkite.com/sourcegraph/sourcegraph/builds/133499