Deprecate `BUILD` variables

We created a bunch of new variables to replace old BUILD variables in GitLab CI/CD. We should officially deprecate usage of these variables and plan for removing them. These were changed in 9.0, so it makes sense to consider removing them in 10.0, although it looks like we never officially deprecated them. It's not that big of a deal to keep them around until 11.0, but we shouldn't keep them around forever.

See https://docs.gitlab.com/ee/ci/variables/#9-0-renaming

/cc @bikebilly @ayufan