Changed name of WaterHeaterEntityDescription

A naming error of the entity description was found in the Water Heater integration, and we have now renamed WaterHeaterEntityEntityDescription to WaterHeaterEntityDescription. As there were no Core integrations and no custom integrations published on HACS using the entity description, we did not add a deprecation period. The changed entity description will be released in the 2025.1 release.

See details in the core PR: #132888.