Fix RCS Signing BWC Tests by Adding CA cert to FC (#136384)

We have to trust the cert we're signing with. Introduced in
#135674.

Resolves: #136368

Author: jfreden

muted-tests.yml

@@ -621,9 +621,6 @@ tests:
 - class: org.elasticsearch.xpack.restart.FullClusterRestartIT
   method: testDataStreams {cluster=UPGRADED}
   issue: https://github.com/elastic/elasticsearch/issues/136353
-- class: org.elasticsearch.xpack.remotecluster.RemoteClusterSecurityBWCToRCS2ClusterRestIT
-  method: testBwcCCSViaRCS1orRCS2
-  issue: https://github.com/elastic/elasticsearch/issues/136368
 - class: org.elasticsearch.xpack.restart.FullClusterRestartIT
   method: testServiceAccountApiKey {cluster=UPGRADED}
   issue: https://github.com/elastic/elasticsearch/issues/136390

x-pack/plugin/security/qa/multi-cluster/src/javaRestTest/java/org/elasticsearch/xpack/remotecluster/RemoteClusterSecurityBWCToRCS2ClusterRestIT.java

@@ -40,6 +40,8 @@ public class RemoteClusterSecurityBWCToRCS2ClusterRestIT extends AbstractRemoteC
             .setting("xpack.ml.enabled", "false")
             .setting("remote_cluster_server.enabled", "true")
             .setting("remote_cluster.port", "0")
+            .setting("cluster.remote.signing.certificate_authorities", "signing.crt")
+            .configFile("signing.crt", Resource.fromClasspath("signing/signing.crt"))
             .setting("xpack.security.remote_cluster_server.ssl.enabled", "true")
             .setting("xpack.security.remote_cluster_server.ssl.key", "remote-cluster.key")
             .setting("xpack.security.remote_cluster_server.ssl.certificate", "remote-cluster.crt")