electrs: adapt to version 0.9.0

- `waitfornewblock` was previously not included in the public RPC
  whitelist because it's reserved for testing and marked as hidden
  in bitcoind.

- electrs changed its verbosity settings. `-vv` is now the best choice
  for normal usage.

- bitcoind option `dataDirReadableByGroup` is now unused.
  Because it can be valuable for other use cases and implementing
  it is intricate, we're keeping it for now.

- test: keep `nc` connection open because otherwise the electrs
  RPC server would now close the connection before sending a response.

modules/hardware-wallets.nix

@@ -38,7 +38,6 @@ in {
         { assertion = (config.services.bitcoind.disablewallet == null || !config.services.bitcoind.disablewallet);
           message = ''
             Hardware-Wallets are not compatible with bitcoind.disablewallet.
-            Note that this option is active when enabling electrs.high-memory.
           '';
         }
       ];