Beacons Update to did:btcr2 (#242)

Updates reference to did:btc1 to did:btcr2, including:
- BTC1 Beacon to BTCR2 Beacon
- BTC1 Update Announcement to BTCR2 Update Announcement
- BTC1 Update to BTCR2 Update
- Unsecured BTC1 Update to Unsecured BTCR2 Update
- Pending BTC1 Update to Pending BTCR2 Update
- Announced BTC1 Update to Announced BTCR2 Update

Author: jenniemeier