s/campaign/batch change/

Created by: LawnGnome

This is fairly mechanical, although I actually did most of it by hand. The only bit that's really interesting is that I needed to abstract two GraphQL mutations to maintain cross-version compatibility: I did so by introducing a new Operations interface in internal/batchers/graphql that delegates to an appropriate implementation based on our existing "feature flag" quirk detection, and takes that level of detail out of internal/batchers/Service.

We're also renaming the volume workspace Docker image: this is fine, since previous tagged versions will continue to just work. The release automation will be mildly spicy when we do the next src release, as there's only so much one can bench test without actually invoking GitHub Actions; it may be worth cutting a release as soon as we have the documentation up to date while this is (relatively) fresh in my mind, even though it will lift the veil on the great renaming slightly early. (Since everything is aliased, nothing should break in practice.)

This is paired with https://github.com/sourcegraph/sourcegraph/pull/18963, which includes the schema changes required to make this work. Those schema changes are not breaking, but we should nevertheless keep both repos in sync for (I hope) obvious reasons.

Closes sourcegraph/sourcegraph#18940.