Vscode-java: Add support for JDK 15

[sunbos]

[provide a description of the issue]
jdk15 is not support
错误: 加载主类 App 时出现 LinkageError
java.lang.UnsupportedClassVersionError: App (class file version 58.65535) was compiled with preview features that are unsupported. This version of
the Java Runtime only recognizes preview features for class file version 59.65535

Environment

• Operating System:Window10
• JDK version:15
• Visual Studio Code version:1.49.1
• Java extension version:v0.10.0

Steps To Reproduce

1. [step 1]
2. [step 2]

[Please attach a sample project reproducing the error]
[Please attach logs](https://github.com/redhat-developer/vscode-java/wiki/Troubleshooting#enable-logging)

Current Result

Expected Result

Additional Informations

[fbricon]

Please note that Java 15 support has not been released yet. You can manually install the latest CI build with Java 15 support from https://download.jboss.org/jbosstools/jdt.ls/staging/?C=M;O=D, give it a try and report back any issues

[rk0cc]

Please note that Java 15 support has not been released yet. You can manually install the latest CI build with Java 15 support from https://download.jboss.org/jbosstools/jdt.ls/staging/?C=M;O=D, give it a try and report back any issues

Okay, but when it release JDK 15 supported in vs code since I want to use stable version

[fbricon]

Wednesday next week, give or take.

[fbricon]

See https://github.com/redhat-developer/vscode-java/wiki/Enabling-Java-preview-features to learn how to enable preview features