[9.1] Reenable Index Compatibility BWC Tests (#134800)

Similar to #134784 for 9.1.

qa/lucene-index-compatibility/build.gradle

@@ -14,6 +14,7 @@ buildParams.bwcVersions.withLatestReadOnlyIndexCompatible { bwcVersion ->
   tasks.named("javaRestTest").configure {
     systemProperty("tests.minimum.index.compatible", bwcVersion)
     usesBwcDistribution(bwcVersion)
+    enabled = true
   }
 }