MIGRATION-467 Replace deprecated slaveOk logic with mongosh-compatible read preference

[idarowan]

When connected to a secondary, detect whether the shell is mongosh.

• For mongosh, set read preference to primaryPreferred to avoid deprecation warnings.
• For legacy shell, continue to use rs.secondaryOk() if available.
• Remove fallback to slaveOk() and throw explicit error if secondary reads cannot be enabled.

This ensures compatibility with mongosh while preserving legacy shell support. MIGRATION-467