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

apps/jrobin/java/build.xml

@@ -19,6 +19,7 @@
     <!-- only used if not set by a higher build.xml -->
     <property name="javac.compilerargs" value="" />
     <property name="javac.version" value="1.8" />
+    <property name="javac.release" value="8" />
     <property name="manifest.classpath.name" value="Class-Path" />
 
     <target name="compile" depends="depend">
@@ -30,6 +31,7 @@
             deprecation="on"
             source="${javac.version}"
             target="${javac.version}"
+            release="${javac.release}"
             destdir="./build/obj"
             includeAntRuntime="false"
             encoding="UTF-8"