[RUN AS-IF-FOSS] Move read_only/read_write back to Gitlab::Database

What does this MR do?

These methods were scoped to a connection in !65262 (merged). A discussion after the merge request was merged revealed these methods aren't tied to a specific connection, and as such better belong in Gitlab::Database.

I briefly considered moving them to the Gitlab module instead, as the methods don't actually run any database queries. Unfortunately, this isn't possible as this module is loaded too early to prepend it with an EE module.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
  • This is a backstage-only change, so no changelog entry is needed
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.