fix: Remove oneOf bool/obj from settings schema

[Koenkk]

Docs: Koenkk/zigbee2mqtt.io#3331

TODO:

• Test with HA addon

From @Nerivec in #25192

So, this turned into a much bigger job than anticipated...

• Migration system wasn't quite ready for multi-step, fixed that...
• Bumped settings version to 2.1 (provides support for dev users that already have migrated to 2.0.0)
• Removed oneOf for homeassistant, availability and frontend => only object
• Some nice code cleanups here and there with this one
• Added defaults in schema and settings
• Renamed getInternalSettings to getPersistedSettings
• Updated example yaml
• Fix tests

TODO:

• If you want to keep Z2M version in sync with settings version, would need to update to 2.1.0
• If not, probably should rename the log from migration-2.0.x-to-2.1.x.log to migration-2-to-2.1.log (same for tests labels)
• Which is the expected default for homeassistant.status_topic (several occurrences)?
• hass/status
• homeassistant/status