OIP-191: Disable OHM v1 Migration Pathway

## Summary This proposal recommends permanently disabling the OHM v1 → OHM v2 migration pathway, which has been available for over 4 years at the time of writing. The gOHM currently in the migration contract will be burned. *OIP technical note from the author:* Amendment to account for technical implementation: The OIP will be posted to Snapshot on 24 December 13:00 UTC for 4 days. If the proposal passes, the 6 week period referenced will start at the passing of the Snapshot vote, and all...