Update agency-index.json metaschema and description

Signed-off-by: Natalia Luzuriaga <natalia.luzuriaga@cms.hhs.gov>

Author: natalialuzuriaga

schemas/agency-index/schema-2.0.0.json

@@ -1,5 +1,5 @@
 {
-    "$schema": "http://json-schema.org/draft-04/schema#",
+    "$schema": "https://json-schema.org/draft/2020-12/schema",
     "title": "Code.gov Inventory",
     "description": "A federal source code catalog",
     "type": "object",