Add `deprecated=true|{}` to schema

[nikku]

What should we do?

Element templates may be marked as deprecated:

{
  "id": "my-template",
  "deprecated": true
}

But also by providing additional meta-data:

{
  "id": "my-template",
  "deprecated": {
    "message": "Optional message, please migrat to 'FOO BAR'",
    "documentationRef": "https://optional-link-to-migration-documentation"
  }
}

Why should we do it?

Ensure people use deprecated in accordance with our requirements.