bug: add missing conclusion to CheckRunConclusion (#144)

For all possible valid valuse, see 'conclusion' section in https://docs.github.com/en/rest/checks/runs?apiVersion=2022-11-28

Author: vootelerotov

src/main/java/com/spotify/github/v3/checks/CheckRunConclusion.java

@@ -32,5 +32,6 @@ public enum CheckRunConclusion {
   failure,
   neutral,
   success,
+  skipped,
   stale
 }