Don't block trezor use due to old firmware

[jlopp]

I was just testing our open source "sovereign recovery" process for Casa 3-of-5 multisig users and noticed that it no longer works for me due to the "outdated firmware" error. I'm getting this error while trying to use a Model T running firmware version 2.0.7 while the MINIMUM_FIRMWARE_VERSION in the trezor python library being used is 2.1.0

Because Casa's multisig key management app is seedless (users don't store the seed phrases for devices) it's likely that their device firmware may be out of date. This is because firmware updates sometimes wipe the seed from the device, thus requiring the user to perform a key rotation. Also, hardware devices are geographically distributed and may not be used for long periods of time.

Long story short, if a catastrophic event were to befall Casa and our users need to recover funds without using our servers to coordinate signing, they'll need to be able to recreate their wallets using their hardware devices with whatever the currently running firmware version is, and upgrading the firmware may not be an option.

Trezor devs informed me that the minimum version is for security reason rather than compatibility reasons; as noted above in this case it's a geographically distributed multisig wallet, so I think the single device firmware security flaw is not a catastrophic vulnerability - our users tend to use a blend of different hardware devices to eliminate single points of failure.

Related issues:

https://github.com/trezor/python-trezor/issues/378

https://github.com/spesmilo/electrum/issues/5391