Remove `Gitlab::Database#create_connection_pool` and provide LB one

What does this MR do?

Based on: !67057 (merged). Will automatically re-target once the !67057 (merged) is merged.

The primary intent is to remove Gitlab::Database#create_connection_pool and rather ensure that a pool is created in LB context from a db_config of a current pool.

This, since the pool is cloned, also creates a HashConfig indicating a connection specification that can be later used for db_roles.

Conformity

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• 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.