Clarify that old CI_BUILD_* variables are deprecated

In 9.0 we deprecated all the $CI_BUILD_* in favor of $CI_JOB_*, and the announcement was added to the blog post and in the documentation (https://docs.gitlab.com/ee/ci/variables/#predefined-variables-environment-variables).

It is not clear that they are deprecated if you link directly to https://docs.gitlab.com/ee/ci/variables/#9-0-renaming, the notice must be added there too.