[MM-61462] Fix errcheck issues in server/channels/app/post.go (#29205)

Co-authored-by: Ben Schumacher <ben.schumacher@mattermost.com>

server/.golangci.yml

@@ -119,7 +119,6 @@ issues:
         channels/app/plugin_install.go|\
         channels/app/plugin_signature_test.go|\
         channels/app/plugin_test.go|\
-        channels/app/post.go|\
         channels/app/post_helpers_test.go|\
         channels/app/post_test.go|\
         channels/app/security_update_check.go|\