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/presets/secure-node.nix

@@ -35,7 +35,6 @@ in {
     services.bitcoind = {
       enable = true;
       listen = true;
-      dataDirReadableByGroup = mkIf cfg.electrs.high-memory true;
       dbCache = 1000;
     };