Switching back to OpenDTU requires config backup

[schlimmchen]

We describe how to switch to OpenDTU-OnBattery and also how to switch back. However, switching back is not seamless any more, as we at least use the DTU polling interval differently. So one shoul have a backup when switching back and forth. The basics should be fine, i.e., one should be able to acccess the web UI after switching, but one should still restore a config backup when switching back to OpenDTU.

We should also note that an implicit downgrade (switching to either OpenDTU or OpenDTU-OnBattey with a lower upstream version) is never supported.