Mute UpgradeClusterClientYamlTestSuiteIT#indices.create/20_synthetic_source tests (#115925)

Manual mute for: #115910,
#115911,
#115912

These are also failing on 8.16.

Author: n1v0lg

muted-tests.yml

@@ -355,6 +355,15 @@ tests:
 - class: org.elasticsearch.reservedstate.service.FileSettingsServiceTests
   method: testProcessFileChanges
   issue: https://github.com/elastic/elasticsearch/issues/115280
+- class: org.elasticsearch.backwards.MixedClusterClientYamlTestSuiteIT
+  method: test {p0=indices.create/20_synthetic_source/object with unmapped fields}
+  issue: https://github.com/elastic/elasticsearch/issues/115910
+- class: org.elasticsearch.backwards.MixedClusterClientYamlTestSuiteIT
+  method: test {p0=indices.create/20_synthetic_source/empty object with unmapped fields}
+  issue: https://github.com/elastic/elasticsearch/issues/115911
+- class: org.elasticsearch.backwards.MixedClusterClientYamlTestSuiteIT
+  method: test {p0=indices.create/20_synthetic_source/nested object with unmapped fields}
+  issue: https://github.com/elastic/elasticsearch/issues/115912
 
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