Fix database name lost from connection config

What does this MR do?

When the config/database.yml is using a three-tier config with main:, fix a place where we call establish_connection, and lose the database name. This ~bug was first found in !68232 (comment 652631254)

Screenshots or Screencasts (strongly suggested)

How to setup and validate locally (strongly suggested)

1. Setup the config/database.yml to have three-tier style config. It should already be setup so with GDK [1] - if not, run make gitlab/config/database.yml to regenerate it.
2. Run rails console
3. ActiveRecord::Base.connection.pool.db_config.name should return main now.

[1] Here's my config for example:

development:
  main:
    adapter: postgresql
    encoding: unicode
    database: gitlabhq_development
    host: /Users/tkuah/code/ee-gdk/postgresql
    port: 5432
    pool: 10
    prepared_statements: false
    variables:
      statement_timeout: 120s

test: &test
  main:
    adapter: postgresql
    encoding: unicode
    database: gitlabhq_test
    host: /Users/tkuah/code/ee-gdk/postgresql
    port: 5432
    pool: 10
    prepared_statements: false
    variables:
      statement_timeout: 120s

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.)
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.