Stop testing on non-LTS Ember releases before v3.24

package.json

@@ -79,7 +79,7 @@
     ],
     "configPath": "tests/dummy/config",
     "versionCompatibility": {
-      "ember": ">=3.8"
+      "ember": "3.8 || 3.12 || 3.16 || 3.20 || >=3.24"
     }
   }
 }