cmd/{adminapp,adminredirect,googlegolangorg}: update App Engine runtime

Keep them up with the times.

Change-Id: I8f6940b59cac0c70320a796a0c3cfcdef8a2633d
Reviewed-on: https://go-review.googlesource.com/c/website/+/531676
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

cmd/adminapp/app.yaml

@@ -1,5 +1,5 @@
 service: default
-runtime: go120
+runtime: go121
 main: ./cmd/adminapp
 
 env_variables:

cmd/adminredirect/app.yaml

@@ -1,5 +1,5 @@
 service: admin
-runtime: go119
+runtime: go121
 main: ./cmd/adminredirect
 
 env_variables:

cmd/googlegolangorg/app.yaml

@@ -1,3 +1,3 @@
 service: google
-runtime: go119
+runtime: go121
 main: ./cmd/googlegolangorg