Set MR Jar classifier to "mr"

Prevents conflicting with main jar. Buildscript authors should change this themselves if they prefer.
```groovy
tasks.named('multiReleaseJar', Jar) {
    archiveClassifier = ''
}
```
This and many others will be documented in due time.

Author: Jonathing

src/main/java/net/minecraftforge/multi_release/gradle/internal/MultiReleaseContainerImpl.java

@@ -75,8 +75,9 @@ private TaskProvider<Jar> createJar() {
 
             task.with(jar.get());
             task.setManifest(jar.get().getManifest());
-
             task.manifest(Closures.<Manifest>consumer(manifest -> manifest.getAttributes().put("Multi-Release", "true")));
+
+            task.getArchiveClassifier().convention("mr");
         });
 
         getProject().getTasks().named(LifecycleBasePlugin.ASSEMBLE_TASK_NAME).configure(task -> task.dependsOn(ret));