Update schema builder to use JSON draft 2020-12

[mirpedrol]

Description of feature

nf-validation v2.0 uses the latest JSON Schema draft 2020-12

Prior to re-writing the nf-core schema builder, we should update the basis to make it compatible with the new version of nf-validation. That is:

• Changing the schema specification form http://json-schema.org/draft-07/schema to https://json-schema.org/draft/2020-12/schema
• Changing the keyword definitions to defs

nf-validation migration docs

[ewels]

Should be able to do this with a quick string regex on the returned JSON from the web builder, before saving locally.

[alvaromartmart]

Isn't $defs the correct keyword in the new schema and not defs? https://json-schema.org/draft/2019-09/release-notes#keyword-changes