Resolve "GitLab 9.0 to 9.2 is incompatible with PostgreSQL 9.1"

What does this MR do?

Fixes a postgresql incompatibility, allowing migrations to run against postgresql 9.1

Are there points in the code the reviewer needs to double check?

Adding remove_concurrent_index_by_name was annoying, but the alternative was to modify remove_concurrent_index and all migrations currently using it - about 150 sites.

I prefer the smaller diff for now; we can refactor later in an MR that won't need picking into stable.

We need to test against postgres 9.1 if we're supporting it; I'll update https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11677 to set the version back to 9.2 once this is merged.

Why was this MR needed?

Upgrading to GitLab 9.2, and downgrading to GitLab 9.0, are broken on postgresql 9.1 at the moment, which is a supported version.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Related to #32851 (closed)