Change the default authentication method to SCRAM-SHA-256

[cbandy]

PostgreSQL has stored passwords as SCRAM-SHA-256 since PostgreSQL 14. PGO has stored passwords as SCRAM-SHA-256 since PostgreSQL 10.

Checklist:

• Have you added an explanation of what your changes do and why you'd like them to be included?
• Have you updated or added documentation for the change, as applicable?
• Have you tested your changes on all related environments with successful results, as applicable?
  • Have you added automated tests?

Type of Changes:

• Other

What is the current behavior (link to any open issues here)?

When not configured, PostgresCluster allows connections from users with an MD5 password stored in the database.

PGO has configured Postgres to hash new and rotated passwords with SCRAM for years.

What is the new behavior (if this is a feature change)?

• Breaking change (fix or feature that would cause existing functionality to change)

When not configured, PostgresCluster requires users to have a SCRAM password in the database.

Passwords stored as MD5 are the exception now. In those cases, one can use spec.authentication.rules and spec.config.parameters to downgrade to MD5.

Other Information:

Issue: PGO-2290
See: https://www.postgresql.org/docs/current/auth-password.html