[MM-61464] Fix errcheck issues in server/channels/app/post_metadata.go (#29204)

Co-authored-by: Mattermost Build <build@mattermost.com>

server/.golangci.yml

@@ -127,7 +127,6 @@ issues:
         channels/app/plugin_test.go|\
         channels/app/post.go|\
         channels/app/post_helpers_test.go|\
-        channels/app/post_metadata.go|\
         channels/app/post_test.go|\
         channels/app/security_update_check.go|\
         channels/app/server.go|\