Fix warm-cache groups runtime error

Running warm-cache groups would cause the following runtime exception:

```
[2023-03-09 17:53:54,398] [SSH warm-cache groups  (admin)] ERROR
com.google.gerrit.sshd.BaseCommand : Internal server error (user admin
account 1000000) during warm-cache groups
groovy.lang.MissingMethodException: No signature of method:
com.google.gerrit.server.account.GroupIncludeCacheImpl.subgroupsOf() is
applicable for argument types:
(com.google.gerrit.reviewdb.client.AccountGroup$UUID) values:
```

The subgroupsOf method is no longer available, also the cache.get()
returns an optional rather than null.

Change-Id: I950c42a42d34adc632bf5cbe01683def0236bc9a
diff --git a/admin/warm-cache-1.0.groovy b/admin/warm-cache-1.0.groovy
index f5f9955..46c34f1 100644
--- a/admin/warm-cache-1.0.groovy
+++ b/admin/warm-cache-1.0.groovy
@@ -89,12 +89,12 @@
     def groupsLoaded = 0
 
     for (groupUuid in allGroupsUuids) {
-      groupIncludeCache.subgroupsOf(groupUuid)
       groupIncludeCache.parentGroupsOf(groupUuid)
       def group = groupCache.get(groupUuid)
-      if(group != null) {
-        groupCache.get(group.getNameKey())
-        groupCache.get(group.getId())
+
+      if(group.isPresent()) {
+        groupCache.get(group.get().getNameKey())
+        groupCache.get(group.get().getId())
       }
       groupsLoaded++