Use Java 21 by default in the Deploying to Google Cloud guide

(cherry picked from commit 4d68e63)

docs/src/main/asciidoc/deploying-to-google-cloud.adoc

@@ -60,14 +60,14 @@ Then, you will need to create a `src/main/appengine/app.yaml` file, let's keep i
 
 [source, yaml]
 ----
-runtime: java17
+runtime: java21
 ----
 
 This will create a default service for your App Engine application.
 
 [NOTE]
 ====
-You can also use another Java runtime supported by App Engine, for example, for Java 21, use `runtime: java21` instead.
+You can also use another Java runtime supported by App Engine, for example, for Java 17, use `runtime: java17` instead.
 ====
 
 App Engine Standard does not support the default Quarkus' specific packaging layout, therefore, you must set up your application to be packaged as an uber-jar via your `application.properties` file: