Consistent JSON schema version for core and extensions

The JSON Schema version of the core specification has been updated from `draft-04` to `draft/2020-12` (via https://github.com/KhronosGroup/glTF/pull/2031 and https://github.com/KhronosGroup/glTF/pull/2034).

This raises some questions about the consistency of the specification. The core specification now uses `draft/2020-12`, whereas the extensions usually use `draft-04`. Without any guidelines or further actions, this could cause considerable fragmentation and inconsistencies.

Will the existing extension specifications be updated to also use `draft/2020-12`? (Who will do that, and how?)

Is there any guideline for the schema versions that extension specifications should use? (The section at https://github.com/KhronosGroup/glTF/tree/main/extensions#creating-extensions does not say anything).

Start with the extension guidance at extensions/#creating-extensions and compare its stated guidance with the core specification's draft/2020-12 schema version and the extensions' draft-04 usage. Done means the project has a decided, documented schema-version policy and a clear plan for applying it to existing extensions.

Written by the indexing model from the issue text.