cmd/golangorg: update App Engine runtime to Go 1.19

Now that it's available, start deploying with it.

For golang/go#51800.

Change-Id: I11c4dc131e33f192e9d7ea1fb5a154fc3422dece
Reviewed-on: https://go-review.googlesource.com/c/website/+/456521
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>

cmd/golangorg/app.yaml

@@ -2,7 +2,7 @@
 # Do not deploy directly.
 
 service: default
-runtime: go116
+runtime: go119
 main: ./cmd/golangorg
 
 env_variables: