lower classfile version to Java 8

[kram617]

We are unable to use json-typedef-java with our current Java 8 projects due to the latest 0.2.2 version being compiled against Java 15. Since it doesn't appear this library depends on Java features present in newer versions in Java, can this project be rebuilt to lower the classfile version to be compatible with Java 8?

wget https://repo1.maven.org/maven2/com/jsontypedef/jtd/jtd/0.2.2/jtd-0.2.2.jar
jar xf jtd-0.2.2.jar
file com/jsontypedef/jtd/Type.class
com/jsontypedef/jtd/Type.class: compiled Java class data, version 59.0