"projects/playground": Use security.[Local/Remote]BlessingNames

CL 8243 depecrated security.BlessingNames and instead
introduced security.[Local/Remote]BlessingNames. This CL
updates the playground repo in accordance to this change.

Change-Id: I7c9a27b4da9d96aafc2a785d152b66e336ff0398
diff --git a/go/src/playground/testdata/src/pong/pong.go b/go/src/playground/testdata/src/pong/pong.go
index 8764c0a..d96dcc5 100644
--- a/go/src/playground/testdata/src/pong/pong.go
+++ b/go/src/playground/testdata/src/pong/pong.go
@@ -18,7 +18,7 @@
 type pongd struct{}
 
 func (f *pongd) Ping(call rpc.ServerCall, message string) (result string, err error) {
-	remote, _ := security.BlessingNames(call.Context(), security.CallSideRemote)
+	remote, _ := security.RemoteBlessingNames(call.Context())
 	fmt.Printf("%v: %q\n", remote, message)
 	return "PONG", nil
 }