[CI] Mute Ml rolling upgrade test for mixed cluster too

It can fail in either the mixed cluster or the upgraded cluster,
so it needs to be muted in both.

Tracked by elastic#30982

Author: droberts195

x-pack/qa/rolling-upgrade/src/test/resources/rest-api-spec/test/mixed_cluster/30_ml_jobs_crud.yml

@@ -7,6 +7,9 @@ setup:
 
 ---
 "Test get old cluster job":
+  - skip:
+      version: "all"
+      reason: "@AwaitsFix: https://github.com/elastic/elasticsearch/issues/30982"
   - do:
       xpack.ml.get_jobs:
         job_id: old-cluster-job