[8.x] Change backwards test configuration to use trial license (#115226) (#115244)

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

* Change backwards test configuration to use trial license

* unmute

(cherry picked from commit 5e761fe4d0733c3cdc6d60c5f955dcd9086c1843)

# Conflicts:
#	muted-tests.yml

* Update muted-tests.yml

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"))) {