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/obsolete-options.nix

@@ -26,5 +26,12 @@ in {
 
     (mkRenamedAnnounceTorOption "clightning")
     (mkRenamedAnnounceTorOption "lnd")
+
+    # 0.0.53
+    (mkRemovedOptionModule [ "services" "electrs" "high-memory" ] ''
+      This option is no longer supported by electrs 0.9.0. Electrs now always uses
+      bitcoin peer connections for syncing blocks. This performs well on low and high
+      memory systems.
+    '')
   ];
 }