[2.3.1] Fix $schema identifier in JSON Schema

[Bo98]

While pre-2019 JSON Schema drafts had a trailing #, the 2019-09 draft does not:

The identifier for Draft 2019-09 is https://json-schema.org/draft/2019-09/schema.

https://json-schema.org/understanding-json-schema/reference/schema

Many validators locally store rules for the different schemas and match them by simple string comparison rather than fetching the remote schema, so the trailing # was causing issues here.

This issue is specific to the 2.3.1 branch - the identifier in 2.3.0 is correct.