Change backwards test configuration to use trial license (elastic#115226)

* Change backwards test configuration to use trial license

* unmute

Author: kkrik-es

muted-tests.yml

@@ -346,8 +346,6 @@ tests:
   issue: https://github.com/elastic/elasticsearch/issues/115129
 - class: org.elasticsearch.xpack.esql.ccq.MultiClusterSpecIT
   issue: https://github.com/elastic/elasticsearch/issues/115135
-- class: org.elasticsearch.backwards.MixedClusterClientYamlTestSuiteIT
-  issue: https://github.com/elastic/elasticsearch/issues/115213
 - class: org.elasticsearch.xpack.esql.expression.function.scalar.string.ReverseTests
   method: testEvaluateInManyThreads {TestCase=<long unicode TEXT>}
   issue: https://github.com/elastic/elasticsearch/issues/115227

qa/mixed-cluster/build.gradle

@@ -71,6 +71,7 @@ BuildParams.bwcVersions.withWireCompatible { bwcVersion, baseName ->
       numberOfNodes = 4
       setting 'path.repo', "${buildDir}/cluster/shared/repo/${baseName}"
       setting 'xpack.security.enabled', 'false'
+      setting "xpack.license.self_generated.type", "trial"
       /* There is a chance we have more master changes than "normal", so to avoid this test from failing,
        we increase the threshold (as this purpose of this test isn't to test that specific indicator). */
       if (bwcVersion.onOrAfter(Version.fromString("8.4.0"))) {