Source Github: Add undeclared columns to spec (#24523)

Signed-off-by: Sergey Chvalyuk <grubberr@gmail.com>

airbyte-integrations/connectors/source-github/source_github/schemas/projects.json

@@ -3,7 +3,7 @@
   "type": "object",
   "properties": {
     "repository": {
-      "type": ["string"]
+      "type": "string"
     },
     "owner_url": {
       "type": ["null", "string"]
@@ -39,11 +39,11 @@
       "$ref": "user.json"
     },
     "created_at": {
-      "type": ["null", "string"],
+      "type": "string",
       "format": "date-time"
     },
     "updated_at": {
-      "type": ["null", "string"],
+      "type": "string",
       "format": "date-time"
     }
   }