Add paperweight-mappings-namespace attribute

universal/build.gradle.kts

@@ -14,6 +14,9 @@ dependencies {
 
 tasks {
     shadowJar {
+        manifest {
+            attributes["paperweight-mappings-namespace"] = "mojang"
+        }
         archiveFileName.set("ViaBackwards-${project.version}.jar")
         destinationDirectory.set(rootProject.projectDir.resolve("build/libs"))
     }