Merge #21417: Misc external signer improvement and HWI 2 support

57ff5a42ab doc: specify minimum HWI version (Sjors Provoost)
03308b2bfa rpc: don't require wallet for enumeratesigners (Sjors Provoost)

Pull request description:

  HWI just released 2.0. See https://github.com/bitcoin-core/HWI/releases/tag/2.0.0

  As of #16546 we already rely on features that are in 2.0 and not in the previous 1.* releases:
  * `--chain` param

  This shouldn't be a problem, because HWI 2.0 has been released before we release v22.

  Misc improvements:
  * document that HWI 2.0 is required
  * drop wallet requirement for `enumeratesigners`

ACKs for top commit:
  achow101:
    Code Review ACK 57ff5a42ab

Tree-SHA512: 3fb6ba20894e52a116f89525a5f5a1f61d363ccd904e1cffd0e6d095640fc6d2edf0388cd6ae20f83bbc31e5f458255ec090b6e823798d426eba3e45b4336bf9

src/wallet/rpcsigner.cpp

@@ -32,9 +32,6 @@ static RPCHelpMan enumeratesigners()
         },
         RPCExamples{""},
         [](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue {
-            std::shared_ptr<CWallet> const wallet = GetWalletForJSONRPCRequest(request);
-            if (!wallet) return NullUniValue;
-
             const std::string command = gArgs.GetArg("-signer", "");
             if (command == "") throw JSONRPCError(RPC_WALLET_ERROR, "Error: restart bitcoind with -signer=<cmd>");
             std::string chain = gArgs.GetChainName();