Update JDK version to 1.8

[yanyir]

• sets source and target in pom.xml to Java 8

Please let me know if there is anything else I need to do :)

[yanyir]

@hengyunabc 您好，我目前只改了pom.xml，想请问一下还需要做什么，如果可以的话请给些建议，谢谢！