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-docs/src/main/asciidoc/configuration-format.adoc

@@ -167,15 +167,15 @@ UI. This also drives how the meta-data for your instance are generated and tooli
 support is meant to react to that.
 
 For instance, if you want your groupId to default to `org.acme` and the
-`javaVersions` to only be `1.7` and `1.8` you would write the following config:
+`javaVersions` to only be `1.8` and `10` you would write the following config:
 
 ```yml
 initializr:
   groupId:
     value: org.acme
   javaVersions:
+    - id: 10
+      default: false
     - id: 1.8
       default: true
-    - id: 1.7
-      default: false
 ```

initializr-docs/src/main/asciidoc/user-guide.adoc

@@ -142,7 +142,7 @@ https://start.spring.io/#!language=kotlin&type=gradle-project
 The following attributes are supported:
 
 * Programming language: `language` (`java`, `groovy` or `kotlin`)
-* Java version: `javaVersion` (`1.6`, `1.7`, `1.8`)
+* Java version: `javaVersion` (`1.8`, `10`)
 * Project type: `type` (`maven-project`, `gradle-project`)
 * Packaging: `packaging` (`jar`, `war`)
 * Group: `groupId`