PR 16911 PostgreSQL: Add system_identifier breaks postgres.can_connect against PostgreSQL 9.2 systems

[anordby]

In RHEL 7 after upgrading datadog-agent to version 7.53.0-1, postgres.can_connect fails against PostgreSQL 9.2 systems due to above mentioned change utilizing unsupported pg_control_system:

Apr 30 12:50:04 no000010sladm1 agent: 2024-04-30 12:50:04 CEST | CORE | ERROR | (pkg/collector/python/datadog_agent.go:129 in LogMessage) | postgres:28e87c56525f3d28 | (postgres.py:239) | Unhandled exception while using database connection ecom_order_management
Apr 30 12:50:04 no000010sladm1 agent: Traceback (most recent call last):
Apr 30 12:50:04 no000010sladm1 agent: File "/opt/datadog-agent/embedded/lib/python3.11/site-packages/datadog_checks/postgres/postgres.py", line 224, in db
Apr 30 12:50:04 no000010sladm1 agent: yield self._db
Apr 30 12:50:04 no000010sladm1 agent: File "/opt/datadog-agent/embedded/lib/python3.11/site-packages/datadog_checks/postgres/postgres.py", line 207, in execute_query_raw
Apr 30 12:50:04 no000010sladm1 agent: cursor.execute(query)
Apr 30 12:50:04 no000010sladm1 agent: psycopg2.errors.UndefinedFunction: function pg_control_checkpoint() does not existApr 30 12:50:04 no000010sladm1 agent: LINE 4: FROM pg_control_checkpoint();
Apr 30 12:50:04 no000010sladm1 agent: ^
Apr 30 12:50:04 no000010sladm1 agent: HINT: No function matches the given name and argument types. You might need to add explicit type casts.
Apr 30 12:50:04 no000010sladm1 agent: 2024-04-30 12:50:04 CEST | CORE | ERROR | (pkg/collector/python/datadog_agent.go:129 in LogMessage) | - | (core.py:94) | Error querying pg_control_checkpoint: function pg_control_checkpoint() does not exist
Apr 30 12:50:04 no000010sladm1 agent: LINE 4: FROM pg_control_checkpoint();
Apr 30 12:50:04 no000010sladm1 agent: ^

The change causing this seems to be: #16911

Additional environment details (Operating System, Cloud provider, etc):

RHEL 7, x64

Steps to reproduce the issue:

1. Install datadog-agent version 7.53.0-1
2. Setup postgres monitoring of PostgreSQL 9.2 server with above datadog-agent version
3. See that it fails!

Additional information you deem important (e.g. issue happens only occasionally):

Happens always.