FIX: Don't force show topic map on non-first posts (#19302)

By default, the topic map in the OP shows only if there are replies.
Some themes may want to show it at all times, and to do so, they can
use the API via `api.includePostAttributes('topicMap');`.

But this was including the topic map in every post. This change ensures
that attribute is only set for the first post (and it only affects that
API endpoint).

app/assets/javascripts/discourse/app/lib/transform-post.js

@@ -178,7 +178,7 @@ export default function transformPost(
   }
 
   const showTopicMap =
-    _additionalAttributes.includes("topicMap") ||
+    (_additionalAttributes.includes("topicMap") && post.post_number === 1) ||
     showPMMap ||
     (post.post_number === 1 &&
       topic.archetype === "regular" &&