Remove deprecated `go` constraint

[jamietanna]

As we should use golang instead, if we're using it to manage the actuall tool dependency.

[jamietanna]

That being said, this is blocked by #42601 as we're not currently wiring that in

[jamietanna]

Will need migrations: #42589 (review)

[jamietanna]

We may want to fix this sooner than later: https://github.com/search?q=+path%3Arenovate.json+constraints+%22%5C%22go%5C%22%22&type=code shows a lot of usage