Remove deprecate call to jruby 1.9

 With the move to JRuby9k which use the 2.3 syntax, this make the
 following constant deprecated

 ```
 config_19.setCompatVersion(CompatVersion.RUBY1_9);
 ```

Fixes elastic#7446

logstash-core/src/test/java/org/logstash/TestBase.java

@@ -15,9 +15,8 @@ public abstract class TestBase {
     public void setUp() throws Exception {
         if (setupDone) return;
 
-        RubyInstanceConfig config_19 = new RubyInstanceConfig();
-        config_19.setCompatVersion(CompatVersion.RUBY1_9);
-        ruby = Ruby.newInstance(config_19);
+        RubyInstanceConfig config = new RubyInstanceConfig();
+        ruby = Ruby.newInstance(config);
         RubyBigDecimal.createBigDecimal(ruby); // we need to do 'require "bigdecimal"'
         JrubyTimestampExtLibrary.createTimestamp(ruby);
         setupDone = true;