message: docs: update migration guide
diff: ### Breaking Changes
  - To reduce the bundle sizes of applications using Stacks.js, we are switching from Buffer (a polyfill to match Node.js APIs) to Uint8Arrays (which Buffers use in the background anyway). [Read more...](#buffer-to-uint8array)
  -- To allow message signing on Ledger hardware wallets, we are changing the message signing prefix. [Read more...]
  +- To allow message signing on Ledger hardware wallets, we are changing the message signing prefix. [Read more...](#message-signing-prefix)
  - Post-conditions for NFTs were renamed to be more clear: `Owns` to `DoesNotSend`, `DoesNotOwn` to `Sends`.
  #### Buffer to Uint8Array
  