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

Keep them up with the times.

Change-Id: If9d93df36594dad923fc1aa2ebe2931a76706bac
Reviewed-on: https://go-review.googlesource.com/c/website/+/695997
Auto-Submit: Dmitri Shuralyov <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Robert Findley <[email protected]>

Author: dmitshur

cmd/adminapp/app.yaml

@@ -1,5 +1,5 @@
 service: default
-runtime: go123
+runtime: go125
 main: ./cmd/adminapp
 
 env_variables:

cmd/adminredirect/app.yaml

@@ -1,5 +1,5 @@
 service: admin
-runtime: go123
+runtime: go125
 main: ./cmd/adminredirect
 
 env_variables:

cmd/googlegolangorg/app.yaml

@@ -1,3 +1,3 @@
 service: google
-runtime: go123
+runtime: go125
 main: ./cmd/googlegolangorg