cmd/golangorg: update App Engine runtime to Go 1.21

By now the supported and tested Go versions are Go 1.21 and 1.20.
Update the website deployment to start using the Go 1.21 runtime.
See https://cloud.google.com/appengine/docs/standard/lifecycle/support-schedule#go.

Change-Id: Idc9545ad57ae5de000323bd361b03c13fb918f45
Reviewed-on: https://go-review.googlesource.com/c/website/+/531675
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

cmd/golangorg/app.yaml

@@ -2,7 +2,7 @@
 # Do not deploy directly.
 
 service: default
-runtime: go119
+runtime: go121
 main: ./cmd/golangorg
 
 env_variables: