CEP 14: permit $schema in v1 recipes

[bollwyvl]

Checklist for submitter

• I am submitting a new CEP: Put your title here.
  • I am using the CEP template by creating a copy cep-0000.md named cep-XXXX.md in the root level.
• I am submitting modifications to CEP 14.
  • The PR title reflects the CEP I'm modifying: CEP XX: Amend XYZ.
  • I updated the "Updated" date.
  • I added the link of this PR to the Discussions row.
  • I added or extended the ## Changelog section right above the final "Copyright" section with an item that uses syntax YYYY-MM-DD: Brief explanation of changes.
• Something else: (add your description here).

references

• Add top-level schema fields prefix-dev/recipe-format#82
• Add more standards-based conventions to schema prefix-dev/recipe-format#29
• Allow v1 dollar schema conda-forge/conda-smithy#2617
• feat: Allow $schema top-level field in recipe.yaml prefix-dev/rattler-build#2645

[bollwyvl]

I just tested against rattler-build 0.72.0 and $schema does propagate to info/recipe/recipe.yaml and not to info/recipe/rendered_recipe.yaml. I think this is the correct behavior, so e.g. CEP-40 does not need to be changed at this time.