Disable automatic addition of --enable-preview flag

[provide a description of the issue]

Hi,

Usage of --enable-preview flag during compilation of stand along java file inhabits backward compatibility of generated class file.

It will be more useful if adding this option is left to the user instead of tool automatically adding it.

Thanks

Environment

• Operating System: any
• JDK version: 14
• Visual Studio Code version:
• Java extension version: .49

Steps To Reproduce

1. Compile hello world using jdk 14
2. Run generate class file in jdk 15

[Please attach a sample project reproducing the error]
Please attach logs

Current Result

Expected Result

Additional Informations

复现针对 JDK 14 描述的独立 Java 编译场景，并使用 JDK 15 运行生成的类。检查 vscode-java 的编译路径，找出添加 --enable-preview 的位置；完成标准是该 flag 不再自动添加，并且该行为由适当的测试覆盖。