Update contribution guidelines to require JDK 11 to build

[jzheaux]

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

[manueljordan]

Hi @jzheaux

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

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

[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.

[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

[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.