Uploading JSONSchema with $ref fails

When I upload a schema with a property using `$ref`, I get the error `java.io.FileNotFoundException: http://example.org/abc`.
I would expect that the validator does not resolve this URL and only checks if the schema is valid against the metaschema.

Example:
```json
{
"$schema": "https://json-schema.org/draft/2019-09/schema",
"$id": "http://example.org/idontcare",
"title": "Doesntmatter",
"type": "object",
"properties": {
"abc": {
"$ref": "http://example.org/abc"
}
}
}
```

Hint: I think the problem is that in `JsonUtils::validateJsonSchemaDocument` the same validation method is used as validating a document against a schema which requires resolving the URL.

Start at JsonUtils::validateJsonSchemaDocument and compare how schema documents are validated with document-against-schema validation. Use the provided schema containing a remote $ref to reproduce the FileNotFoundException, then verify that schema validation checks the metaschema without resolving that referenced URL.

Written by the indexing model from the issue text.