now that we have `track.erc721.minting` probably `createdAt` should be renamed to `blockNumber` and put at `track.erc721.minting.blockNumber`

[TimDaub]

• schema/schema.json

  Line 82 in 43c45a3

  "minting": {

• schema/schema.json

  Line 63 in 43c45a3

  "createdAt": {