Remove Java 9 support

To align with Oracle's support policy, this commit only keeps Java 8 and
10. This effectively removes 1.7 as well.

Closes gh-662

initializr-service/src/main/resources/application.yml

@@ -1335,14 +1335,9 @@ initializr:
   javaVersions:
     - id: 10
       default: false
-    - id: 9
-      default: false
     - id: 1.8
       name: 8
       default: true
-    - id: 1.7
-      name: 7
-      default: false
   languages:
     - name: Java
       id: java