0.15: Add old version check to OTA update

[willmmiles]

0.15.2 version of #5057.

Add a field to the OTA metadata structure indicating the oldest base version it's safe to install this update from. This provides a clear path forward in case there are incompatibilities, eg. some case (bootloader compatibility) where 0.16.0 cannot be OTA'd safely from 0.15.2, but a transitional 0.15.3 could potentially arrange the necessary groundwork.

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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[netmindz]

For code like this, how easily could we have unit tests?
Not a blocker for acceptance, just a query

[willmmiles]

For code like this, how easily could we have unit tests? Not a blocker for acceptance, just a query

Honestly I'm not sure. I know platformio has some framework for adding unit tests but I haven't explored it yet.