Spring Security Update contribution guidelines to require JDK 11 to build

As part of updating Spring Security's build to require JDK 11, the contribution guidelines should be updated.

Comment From: manueljordan

Hi @jzheaux

The build.gradle file, should be updated too? Currently, it has:

plugins.withType(JavaPlugin) {
        project.sourceCompatibility='1.8'
    }

Comment From: jzheaux

Good question, @manueljordan. No, this should not be updated as Spring Security 5 guarantees Java 8 source compatibility.

Even though Spring Security may require compilation with JDK 11, it should not require JDK 11 to use it in a project.

Comment From: manueljordan

Even though Spring Security may require compilation with JDK 11, it should not require JDK 11 to use it in a project.

Interesting perspective

Comment From: jzheaux

@manueljordan This is the kind of long-term compatibility that the Spring Framework has become known for. You can see another example of the same thing from when Java 8 APIs started getting introduced. Even though Spring Framework required Java 8 to build it, Java 6 applications could still use it just fine.

Comment From: manueljordan

@jzheaux - Thanks for information and the interesting link, I appreciate that