Fix concentration unit deprecation warnings

[hmmbob]

Summary

• use UnitOfDensity and UnitOfRatio when Home Assistant provides them
• retain compatibility with the declared Home Assistant 2023.2 minimum by falling back to the legacy concentration constants
• update the CO2 unit test to use the compatibility value without importing a deprecated constant

Root cause

Home Assistant 2026.8 warns when integrations import CONCENTRATION_MICROGRAMS_PER_CUBIC_METER or CONCENTRATION_PARTS_PER_MILLION. Their replacements were introduced in Home Assistant 2026.7, while SonoffLAN still declares and enforces Home Assistant 2023.2 as its minimum version.

The feature-detection shim avoids touching the deprecated constants on current Home Assistant versions without dropping support for older installations.

Fixes #1849.

Validation

• full test suite on Home Assistant 2026.8.0b2 / Python 3.14: 94 passed
• fresh Sonoff sensor import on Home Assistant 2026.8.0b2 with no concentration deprecation warning
• legacy and current import paths exercised independently
• changed files parsed successfully using the Python 3.10 grammar required by Home Assistant 2023.2