Ensure license headers match the Cargo manifest licenses (#5776)

- Closes https://github.com/paritytech/license-scanner/issues/44
- Silent because only comments are changed in the crates.

## What's inside

First, we change the file traversal mechanism from shell globbing to
walking through files which happens inside the `license-scanner` - so it
has knowledge about directory structure and can correlate files with
corresponding Cargo manifest.

Next, added `MIT-0` and `Unlicense` to the allowed list of licenses.
`Unlicense` appears only once across {Substrate,Cumulus,Polkadot} - in
`penpal-runtime`.

Finally, updated headers in files that do not match the corresponding
manifest license.

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Co-authored-by: cornholio <0@mcornholio.ru>