Unmuting MixedClusterClientYamlTestSuiteIT (#120462)

Relates to #119806

Author: cbuescher

muted-tests.yml

@@ -450,8 +450,6 @@ tests:
 - class: org.elasticsearch.xpack.migrate.action.ReindexDatastreamIndexTransportActionIT
   method: testTsdbStartEndSet
   issue: https://github.com/elastic/elasticsearch/issues/120314
-- class: org.elasticsearch.backwards.MixedClusterClientYamlTestSuiteIT
-  issue: https://github.com/elastic/elasticsearch/issues/119806
 - class: org.elasticsearch.test.rest.yaml.CcsCommonYamlTestSuiteIT
   method: test {p0=search.vectors/41_knn_search_bbq_hnsw/Vector rescoring has same scoring as exact search for kNN section}
   issue: https://github.com/elastic/elasticsearch/issues/120441