Summary:
This diff removes the `TEST_yb_enable_query_diagnostics` and add a non runtime preview flag `ysql_yb_enable_query_diagnostics`.
Jira: DB-14486

Test Plan: ./yb_build.sh --java-test 'org.yb.pgsql.TestYbQueryDiagnostics'

Reviewers: asaha

Reviewed By: asaha

Subscribers: hbhanawat, yql

Differential Revision: https://phorge.dev.yugabyte.com/D40637