only build with JDK8

Author: chrjohn

.junie/guidelines.md

@@ -21,6 +21,7 @@ QuickFIX/J is a full-featured messaging engine for the FIX protocol. It is a 100
    ```
 
 2. Build the project:
+   Junie, please make sure to only use JDK 8 for the build.
    ```
    ./mvnw clean install -Dmaven.javadoc.skip=true -PskipBundlePlugin,minimal-fix-latest
    ```