Fix telemetry timestamp and update deprecated HA APIs

[LukaszP2]

Summary

This PR updates the Sonic integration for newer Home Assistant versions.

Changes

• Replace the deprecated/removed pytz usage with zoneinfo.ZoneInfo.
• Handle missing telemetry timestamps safely.
• Return a timezone-aware datetime from the telemetry timestamp sensor.
• Use Home Assistant's standard L/min unit for water flow rate.
• Replace async_timeout with Python's built-in asyncio.timeout.
• Remove redundant single-task asyncio.gather() calls.
• Run Sonic device details and telemetry requests concurrently.
• Fix PropertyEntity.async_update() to refresh the device coordinator.
• Improve property availability handling.

Problem

After updating Home Assistant, the telemetry timestamp sensor failed with:

NameError: name 'pytz' is not defined

This caused sensor.telemetry_data_timestamp to become unavailable and generated repeated update listener errors.

Testing

Tested on Home Assistant 2026.8.x.

After the fix:

• Telemetry Data Timestamp is available and updates correctly.
• Water Flow Rate reports L/min.
• The previous pytz exception no longer occurs.
• Integration loads successfully after a full Home Assistant restart.

Compatibility

No changes to the Sonic API or device protocol are required.