Resolve "postgresql 10 support for GitLab"

What does this MR do?

Adds PostgreSQL 10 support to ActiveRecord 4.2 (and so GitLab)

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

I don't understand this code at all, I just filched it from https://github.com/rails/rails/issues/28780#issuecomment-354868174

Why was this MR needed?

ActiveRecord 4.2 doesn't work with pg 10, but various places and people want to use it. If the fix is as minor as that suggested, why not add it?

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
• Review
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

Closes #42047 (closed)