chore: require Java 8

BREAKING CHANGE: the required Java runtime environment is now Java 8 or greater

pom.xml

@@ -162,7 +162,7 @@
     </distributionManagement>
 
     <properties>
-        <java.version>1.7</java.version>
+        <java.version>1.8</java.version>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
         <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
         <maven.build.timestamp.format>yyyy-MM-dd</maven.build.timestamp.format>