An incoming change to defining manually configured MQTT entities might require you to change some lines of YAML in your configuration. MQTT entities will no longer be configured directly under the respective platform keys (e.g., fan, light, switch, etc) and will instead require the MQTT platform key.
If the last two sentences are leaving you confused, the following two code snippets should help you understand what needs to be done as of Home Assistant Core 2022.6. Currently, a manually configured MQTT entity might look like this: