Skips synonym test in mixed cluster bwc tests instead of YAML rest compat (elastic#119044)

(cherry picked from commit cc0e53e)

# Conflicts:
#	muted-tests.yml
#	qa/mixed-cluster/build.gradle

Author: carlosdelest

qa/mixed-cluster/build.gradle

@@ -57,6 +57,9 @@ excludeList.add('cluster.desired_nodes/11_old_format/Test node version must have
 excludeList.add('cluster.desired_nodes/11_old_format/Test node version can not be null')
 excludeList.add('cluster.desired_nodes/20_dry_run/Test validation works for dry run updates')
 
+// Can't work until auto-expand replicas is 0-1 for synonyms index
+excludeList.add("synonyms/90_synonyms_reloading_for_synset/Reload analyzers for specific synonym set")
+
 BuildParams.bwcVersions.withWireCompatible { bwcVersion, baseName ->
 
   if (bwcVersion != VersionProperties.getElasticsearchVersion()) {