Rename the fat jar inside the shadow distribution to openrs2.jar

This removes all the extra cruft (like the whole artifact ID and the
version).

Signed-off-by: Graham <gpe@openrs2.dev>

all/build.gradle.kts

@@ -25,6 +25,8 @@ dependencies {
 }
 
 tasks.shadowJar {
+    archiveFileName.set("openrs2.jar")
+
     minimize {
         exclude(dependency("org.jetbrains.kotlin:kotlin-reflect"))
     }