explicitly target 1.8 jdk for source

Author: Stephen Powis

pom.xml

@@ -83,6 +83,8 @@
     <!-- defined properties -->
     <properties>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+        <maven.compiler.target>1.8</maven.compiler.target>
+        <maven.compiler.source>1.8</maven.compiler.source>
 
         <!-- Define which kafka version -->
         <kafkaVersion>2.8.0</kafkaVersion>