Silence a Cargo warning by renaming .cargo/config

Renamed all .cargo/config Cargo configuration files to
.cargo/config.toml to silence Cargo warnings like:

warning: `<SOME PATH>/discovery/microbit/src/03-setup/.cargo/config` is deprecated in favor of `config.toml`
note: if you need to support cargo 1.38 or earlier, you can symlink `config` to `config.toml`

Author: nikgul