(…and remove extraneous `stringify_keys` - enqueue already does
`deep_stringify_keys`)
The warning in question:
```
Deprecation notice: Jobs::RunProblemCheck was enqueued with argument values which do not cleanly serialize to/from JSON. This means that the job will be run with slightly different values than the ones supplied to `enqueue`. Argument values should be strings, booleans, numbers, or nil (or arrays/hashes of those value types). (deprecated since Discourse 2.9) (removal in Discourse 3.0)
```