[Test] RCS migration for Transform and CCR jobs (#96725)

This PRs adds migration tests for transform and CCR jobs from using RCS
1.0 to 2.0 then back to 1.0 again. It is not intended to cover all
possible scenarios, but rather a proof to show migration is possible in
at least some cases.

Note the migration strategy here is to update remote cluster settings in
place, i.e. reuse the same cluster alias and change its definition.
Restart is required because a node needs to the cross-cluster API key
form the keystore.

Author: ywangd

test/test-clusters/src/main/java/org/elasticsearch/test/cluster/MutableSettingsProvider.java

@@ -25,6 +25,10 @@ public void put(String setting, String value) {
         settings.put(setting, value);
     }
 
+    public void remove(String setting) {
+        settings.remove(setting);
+    }
+
     public void clear() {
         settings.clear();
     }