Require env var for self-managed decomposition

What does this MR do and why?

We want to make two database connections the default. However, a typo in the database: fields means user can accidentally use two databases (decomposed) instead of two database connection.

We add an env var requirement to prevent such a mistake.

Related issue: #387897 (closed)

• Update docs https://docs.gitlab.com/ee/administration/postgresql/multiple_databases.html

Screenshots or screen recordings

This is what you see if you try to setup two databases without the env var

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.