Rename use_replica_if_possible to fallback_to_replicas_for_ambiguous_queries

What does this MR do?

The first part of https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/1005. Originally, use_replica_if_possible was introduced in !56476 (merged).

This MR is to rename use_replica_if_possible to fallback_to_replicas_for_ambiguous_queries. This is straight-forward, just renaming, no logic changing.

After the change, the name fallback_to_replicas_for_ambigiuos_queries reflects the indication that the ambiguous SQL statements from anywhere inside this block should use a replica. The ambiguous statements include:

• Transactions.
• Custom queries (via exec_query, execute, etc.)
• In-flight connection configuration change (SET LOCAL statement_timeout = 5000)

This is a weak enforcement. This helper incorporates well with primary stickiness:

• If the queries are about to write
• The current session already performed writes
• It prefers to use primary, aka, use_primary or use_primary! were called

Screenshots (strongly suggested)

This is an internal change only. No need to attach a screenshot.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • [-] I have included a changelog entry.
  • I have not included a changelog entry because this MR is to rename an internal change at DB load balancing layer.
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• [-] Style guides
• Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done