go.work should be in .gitignore

[kocubinski]

go.work should not be checked into source control. A committed go.work is a statement that the checked in source trees of every module mentioned therein are compatible in build and test phases, locally and in CI. This is not true. We assert that module versions are compatible, not the current state of the source tree.

[julienrbrt]

I agree, this currently causes problems in where PR need to be larger only to satisfy the go.work (cf #12797).
I think disabling it in CI only (like we tried to do for keeping it #12951) is confusing.
The go.work is however very useful. We should make it easy for devs to use it (by having a go.work.example) and have an explanation that when two go modules are modified two PR needs to be created, for avoiding that replace land in main.