Remove support for datastore versions before Sodium SR1

Our general compatibility contract works out to requiring adjacent version to upgrade, yet we have kept historical versions around for multiple years – i.e. we have not removed compatibility since June 2016, when we pruned version earlier than Boron.

We currently support:

• Boron since January 2016
• Fluorine since June 2018
• Neon SR2 since June 2019
• Sodium SR1 since September 2019
• Phosphorus since July 2021

Of these, only Sodium SR1+ can support our current data mapping natively, as older software did not have yang.common.Uint8 and friends.

Remove support for talking to anything older than Sodium SR1, which retains a ~2year backwards compatibility window.