MM-31721: Fix Cache-Control directives (#16763)

We were incorrectly setting the cache control to public
when it should be rather private.

https://mattermost.atlassian.net/browse/MM-31721

```release-notes
Fix Cache-Control headers to instruct that responses may only be cached
on browsers.
```

api4/post.go

@@ -746,7 +746,7 @@ func getFileInfosForPost(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	w.Header().Set("Cache-Control", "max-age=2592000, public")
+	w.Header().Set("Cache-Control", "max-age=2592000, private")
 	w.Header().Set(model.HEADER_ETAG_SERVER, model.GetEtagForFileInfos(infos))
 	w.Write([]byte(model.FileInfosToJson(infos)))
 }