rpc: restore 20s connect timeout for grpc dialer

Changes in cockroachdb#49114 lead to default connect timeout value dropping
to 0 instead of default 20s.

This is not always enough for multiregion clients with hub spoke
network topologies as connections don't have enough time to initially
go through.
This change restores initial value that matches grpc defaults.

Release note (bug fix): Connect timeout for grpc connections is
set to 20s to match pre 20.2 default value.

pkg/rpc/context.go

@@ -72,6 +72,10 @@ const (
 	initialConnWindowSize = initialWindowSize * 16 // for a connection
 )
 
+// GRPC Dialer connection timeout. 20s matches default value that is
+// suppressed when backoff config is provided.
+const minConnectionTimeout = 20 * time.Second
+
 // sourceAddr is the environment-provided local address for outgoing
 // connections.
 var sourceAddr = func() net.Addr {
@@ -967,7 +971,9 @@ func (ctx *Context) grpcDialRaw(
 	// ~second range.
 	backoffConfig := backoff.DefaultConfig
 	backoffConfig.MaxDelay = maxBackoff
-	dialOpts = append(dialOpts, grpc.WithConnectParams(grpc.ConnectParams{Backoff: backoffConfig}))
+	dialOpts = append(dialOpts, grpc.WithConnectParams(grpc.ConnectParams{
+		Backoff:           backoffConfig,
+		MinConnectTimeout: minConnectionTimeout}))
 	dialOpts = append(dialOpts, grpc.WithKeepaliveParams(clientKeepalive))
 	dialOpts = append(dialOpts,
 		grpc.WithInitialWindowSize(initialWindowSize),