MM-56925: Fix unnecessary cache invalidation of pinned post count (#26264)

invalidateCacheForChannelPosts invalidated the pinned post count
and the lastPostTime cache.

But not all endpoints actually pins or unpins the post. That's only
possible via the UpdatePost or PatchPost methods. Therefore, we remove
the unnecessary cache invalidation.

This is also important because this gets called for every single
channel load, and every new post would invalidate this cache.

https://mattermost.atlassian.net/browse/MM-56925
```release-note
NONE
```

server/channels/app/post_acknowledgements.go

@@ -46,11 +46,9 @@ func (a *App) SaveAcknowledgementForPost(c request.CTX, postID, userID string) (
 	}
 
 	// The post is always modified since the UpdateAt always changes
-	a.invalidateCacheForChannelPosts(channel.Id)
+	a.Srv().Store().Post().InvalidateLastPostTimeCache(channel.Id)
 
-	a.Srv().Go(func() {
-		a.sendAcknowledgementEvent(model.WebsocketEventAcknowledgementAdded, acknowledgement, post)
-	})
+	a.sendAcknowledgementEvent(model.WebsocketEventAcknowledgementAdded, acknowledgement, post)
 
 	return acknowledgement, nil
 }
@@ -92,11 +90,9 @@ func (a *App) DeleteAcknowledgementForPost(c request.CTX, postID, userID string)
 	}
 
 	// The post is always modified since the UpdateAt always changes
-	a.invalidateCacheForChannelPosts(channel.Id)
+	a.Srv().Store().Post().InvalidateLastPostTimeCache(channel.Id)
 
-	a.Srv().Go(func() {
-		a.sendAcknowledgementEvent(model.WebsocketEventAcknowledgementRemoved, oldAck, post)
-	})
+	a.sendAcknowledgementEvent(model.WebsocketEventAcknowledgementRemoved, oldAck, post)
 
 	return nil
 }