JSON Schema breaks the API + process specification

[m-mohr]

JSON Schema versions newer than 2020-12 will likely break openEO's use of JSON Schema.
We add a couple of custom keywords (subtype, dimensions, parameters, returns) that will likely be forbidden in the future.
This means tooling must stick to JSON Schema 2020-12 or older until we release openEO API version 2.0.0 which will break all processes and the API spec as we'd need to rename the keywords to something like @subtype, @dimensions, @parameters and @returns or similar.

Related links:

• https://github.com/orgs/json-schema-org/discussions/241
• https://github.com/orgs/json-schema-org/discussions/329
• JSON Schema: unknown keywords opengeospatial/ogcapi-features#855

I have metaschemas around for the keywords, but it will be a pain in the tooling to implement this, if possible at all (ajv supports it, not sure about Python).