Build: Set javac release property (ticket #2775)

Ant version 1.9.8 or higher now required
Drop support for Xenial package build
Fix up BOB build configuration
Fix i2psnark standalone build

installer/tools/java/build.xml

@@ -20,11 +20,13 @@
     <property name="javac.compilerargs" value="" />
     <property name="javac.classpath" value="" />
     <property name="javac.version" value="1.8" />
+    <property name="javac.release" value="8" />
 
     <target name="compile" depends="depend">
         <mkdir dir="./build" />
         <mkdir dir="./build/obj" />
         <javac srcdir="./src" debug="true" source="${javac.version}" target="${javac.version}" deprecation="on"
+            release="${javac.release}"
             includeAntRuntime="false"
             encoding="UTF-8"
             destdir="./build/obj" classpath="${javac.classpath}:../../../build/i2p.jar:../../../core/java/build/gnu-getopt.jar:../../../build/router.jar" >