link to archived/deprecated tools

[gui1117]

in the page: https://docs.substrate.io/maintain/runtime-upgrades/

in the section: Testing migrations

there are 2 links which leads to archived repositories, they should be updated:

- The [Substrate debug kit](https://github.com/paritytech/substrate-debug-kit) includes a [remote externalities](https://github.com/paritytech/substrate-debug-kit/tree/master/remote-externalities) tool that allows storage migration unit testing to be safely performed on live chain data.