Fix HA deprecation warnings for device registry

[tarzan77cz]

Summary

Fixes Home Assistant deprecation warnings reported in the log on every integration load:

1. sw_version / hw_version as int — Webasto API returns firmware version as integer; HA device registry requires strings (will break in HA 2026.12.0).
2. device_registry.async_get_device — replaced with async_get_device_by_identifier (deprecated since HA 2025.x, removed in HA 2027.8.0).

Fix detail (2026-09-11)

The initial PR used an incorrect call signature:

async_get_device_by_identifier(DOMAIN, str(device_id))  # wrong

Correct for HA 2026.9+:

async_get_device_by_identifier((DOMAIN, str(device_id)), entry.entry_id)

entry is passed into _async_update_device_registry_name from _async_setup.

Test plan

• Load integration after restart — no homeassistant.helpers.frame warnings for webastoconnect
• hw_version / sw_version stored as strings in device registry
• Device rename on startup works (_async_update_device_registry_name finds device by identifier + config entry)
• Verified on live ThermoConnect device (rate limit may delay entity load)

Fixes warnings like:

Detected that custom integration 'webastoconnect' passes a non-string value of type int as sw_version...
Detected that custom integration 'webastoconnect' calls `device_registry.async_get_device`...