Follow up on minimal target change to 1.8

Open TODO's from https://github.com/eclipse-jdt/eclipse.jdt.core/pull/2551

Created issues:

• Disabled tests summary ticket: https://github.com/eclipse-jdt/eclipse.jdt.core/issues/2758
• https://github.com/eclipse-jdt/eclipse.jdt.core/issues/2580
• https://github.com/eclipse-jdt/eclipse.jdt.core/issues/2754
• https://github.com/eclipse-jdt/eclipse.jdt.core/issues/2755
• https://github.com/eclipse-jdt/eclipse.jdt.core/issues/2756
• https://github.com/eclipse-jdt/eclipse.jdt.core/issues/2757
• https://github.com/eclipse-jdt/eclipse.jdt.core/issues/2759
• https://github.com/eclipse-jdt/eclipse.jdt.core/issues/2760
• https://github.com/eclipse-jdt/eclipse.jdt.core/issues/2766

Following questions wait for answering by others:

• [Stephan] Should this.currentProject.setOption(JavaCore.COMPILER_PB_RAW_TYPE_REFERENCE, JavaCore.IGNORE); be used in ConverterTestSetup.setUpJavaProject(String, String) ?
• [Stephan]: change on BatchCompilerTest.testBridgeMethodRetention This change probably conflicts with the purpose of this test, @srikanth-sankaran ?
• [Stephan] Change in ASTConverterTestAST8_2.test0612() caught me by surprise so I investigated:
  • the test implies to test AST at JLS8
  • using source from project Converter, Converter15, however, selects the AST level from the project. @srikanth-sankaran , @jarthana does it make any sense have such tests here? Shouldn't tests always sit in a test class corresponding to the used Converter* project, e.g., ASTConverterAST3Test?