Bitcoin 26.0 Mocknet Fix

Bitcoin 26.0 disables the creation of non-descriptor BDB wallets by default. The new wallet format is not currently supported by any of the other UTXO forks and it also requires addresses to be imported differently via descriptors. I propose holding forced upgrade until version 27, which should mark the migratewallet non-experimental (which we can then use to cut over at init).

Th flag to enable the deprecated RPC is added to the mocknet command line here, and will be added to the deployment arguments when the upgrade is applied to the bitcoin-daemon helm chart.