Geo: Handle main attribute in database_geo.yml

What does this MR do?

Resolves #337385 (closed) so we can rerevert omnibus-gitlab!5507 (merged) and unblock the sharding effort as soon as possible.

This has already been manually validated to work with the "main" attribute in geo.staging.gitlab.com #337361 (comment 640387890), as well as in my GDK.

How to setup and validate locally (strongly suggested)

1. In a Geo secondary site
2. Open the config/database_geo.yml file
3. Insert an intermediate main: attribute between the Rails environment (production:, development:, etc) and the connection info. See !65243 (diffs) as an example
4. Restart Rails processes
5. The Rails processes should be able to connect to the Geo tracking database as usual
6. Note that this initializer code is also compatible with the old style of database_geo.yml as well. You can test that by removing the main: attribute.

Does this MR meet the acceptance criteria?

Conformity

• [-] I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• [-] 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.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)