Improve downgrade YAML handling

[3djc]

Is there an existing issue for this feature request?

• [X] I have searched the existing issues

Is your feature request related to a problem?

Quoting @rotorman on Discord: "Numerous users seem to downgrade their radio, e.g. while trying out a release candidate version and then returning to latest (lower) release. The result is more than often a corrupted or unusable YAML files. I am thinking if it would be beneficial to give users a warning if a newer YAML version is detected on start than what current firmware expects. "

Describe the solution you'd like

A couple of alternatives:

• warning at start (imho, useless, users will ignore it)
• warning at start, backup of newer version files, try to work with those
• warning at start, backup of newer version files, work with default
• alert and refuse to start

Describe alternatives you've considered

No response

Additional context

No response