Preserve cluster settings on full restart tests (#33590)

Today the full cluster restart tests do not preserve cluster settings on
restart. This is a mistake because it is not an accurate reflection of
reality, we do not expect users to clear cluster settings when they
perform a full cluster restart. This commit makes it so that all full
cluster restart tests preserve settings on upgrade.

Author: jasontedor

test/framework/src/main/java/org/elasticsearch/upgrades/AbstractFullClusterRestartTestCase.java

@@ -57,4 +57,9 @@ protected boolean preserveTemplatesUponCompletion() {
         return true;
     }
 
+    @Override
+    protected boolean preserveClusterSettings() {
+        return true;
+    }
+
 }