Don't run rest compatibility checks on 7.17 branch

.ci/jobs.t/elastic+elasticsearch+pull-request+rest-compatibility.yml

@@ -19,6 +19,7 @@
           status-context: elasticsearch-ci/rest-compatibility
           cancel-builds-on-update: true
           black-list-target-branches:
+            - 7.17
             - 7.16
             - 7.15
             - 6.8