rpc: remove InsecureSkipVerify from DRPC TLS client config

`InsecureSkipVerify` disables certificate verification and introduces
significant security risks. This commit removes `InsecureSkipVerify` option
from the DRPC TLS client configuration. ServerName in TLS config is used to
ensure the server's certificate matches the expected hostname the client is
connecting to.

gRPC does it here.
https://github.com/grpc/grpc-go/blob/62071420ce2be9eaa916159c1d7609adf3c3aaaa/credentials/tls.go#L110-L120

Epic: CRDB-51459
Issues: #147921
Release note: none

Author: cthumuluru-crdb

pkg/rpc/drpc.go

@@ -79,9 +79,11 @@ func DialDRPC(
 				}
 				// Clone TLS config to avoid modifying a cached TLS config.
 				tlsConfig = tlsConfig.Clone()
-				// TODO(server): remove this hack which is necessary at least in
-				// testing to get TestDRPCSelectQuery to pass.
-				tlsConfig.InsecureSkipVerify = true
+				sn, _, err := net.SplitHostPort(target)
+				if err != nil {
+					return nil, err
+				}
+				tlsConfig.ServerName = sn
 				tlsConn := tls.Client(netConn, tlsConfig)
 				conn = drpcconn.NewWithOptions(tlsConn, opts)
 			}