Upgrade JSON schema version to https://json-schema.org/draft/2020-12/schema

[tadasant]

To match with the official spec.

[piyushbag]

Looking at this. The schema extractor still emits http://json-schema.org/draft-07/schema# while the OpenAPI document already declares the 2020-12 dialect.

Plan: update tools/extract-server-schema to emit https://json-schema.org/draft/2020-12/schema, regenerate the published server schema artifacts, and keep the change scoped to matching the official dialect.

Will open a PR shortly.