XS Add sharding key YAML config to ci_builds & p_ci_builds (1 table)

What we need to do

The ci_builds table has a project_id key, but didn't get the designation automatically a few months ago when the Cells team sent out all the initial MRs.

I'm not sure if there's something blocking this having to do with partitioning.

How to do it

1. Find out if there's any reason we can't add the sharding key designation immediately and have all the automation work.
2. Open up https://gitlab.com/gitlab-org/gitlab/-/blob/master/db/docs/ci_builds.yml

  ---
  table_name: ci_builds
  classes:
  - Ci::Bridge
  - Ci::Build
  - Ci::Processable
  - CommitStatus
  - GenericCommitStatus
  feature_categories:
  - continuous_integration
  description: TODO
  introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/commit/046b28312704f3131e72dcd2dbdacc5264d4aa62
  milestone: '8.0'
  gitlab_schema: gitlab_ci
+ sharding_key:
+   project_id: projects