Reindexing: Limit to GitLab main schema

The following discussion from !42967 (merged) should be addressed:

• @pbair started a discussion: (+1 comment)

  Not sure if I remember this correctly, I think we have assumed there could be other schemas not owned by the app, which is why we chose to prepend the partition schemas with gitlab_

  If that assumption is true, couldn't this pull indexes in a customer schema not controlled by us?