tests: bump java compat level up to 8

Fedora's java package no longer supports 7.

test cases/java/3 args/meson.build

@@ -1,8 +1,8 @@
 project('simplejava', 'java')
 
-add_project_arguments('-target', '1.7', language : 'java')
+add_project_arguments('-target', '1.8', language : 'java')
 
 javaprog = jar('myprog', 'com/mesonbuild/Simple.java',
   main_class : 'com.mesonbuild.Simple',
-  java_args : ['-source', '1.7'])
+  java_args : ['-source', '1.8'])
 test('mytest', javaprog)