Propagate "unavailable" and "unknown" states instead of preserving the last known state

[jasilvanus]

When a source sensor for temperature or humidity becomes unavailable or unknown, currently thermal comfort will not update its values and preserve the latest known value. To reproduce, one can add a simple template sensor, use it as both temperature and humidity in a thermal comfort device, and observe how the thermal comfort device only updates if the source sensor changes to a non-unknown, non-unavailable value.

I think we should instead update to unavailable or unknown in such cases, for the following reasons:

• Currently, automations cannot rely on thermal comfort devices without risking to indefinitely use outdated data in case the underlying sensor (e.g. a battery-powered device) becomes unavailable. Automations could add extra checks against the underlying sensors, but that would be awkward and error-prone.
• Propagating unavailable and unknown lets the thermal comfort device act more consistently with the underlying physical device: If the underlying phyical device could directly report e.g. absolute humidity, that sensor would also become unavailable if the physical device becomes unavailable.

This would be a breaking change, and there could be existing users relying on thermal comfort caching the latest known good value.
An alternative would be add an option to control this behavior.