ajv draft-2020-12 support

Author: epoberezkin

_data/validator-libraries-modern.yml

@@ -134,7 +134,7 @@
     - name: ajv
       url: https://github.com/ajv-validator/ajv
       notes: "for Node.js and browsers - supports [user-defined keywords](https://github.com/ajv-validator/ajv/blob/master/docs/keywords.md) and [$data reference](https://github.com/json-schema-org/json-schema-spec/issues/51)"
-      date-draft: [2019-09]
+      date-draft: [2019-09, 2020-12]
       draft: [7, 6, 4]
       license: MIT
     - name: djv