Crate `proc-macro-warning` adds GPL 3.0 license dependency

[haerdib]

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Description of bug

The newly added proc-macro-warning crate adds a GPL 3.0 dependency to substrate (added in this PR)
To the best of my knowledge (not a lawyer) this violates the GPL v3.0 license.

Steps to reproduce

Noticed after a cargo update via license check in our CI. Check out https://github.com/scs/substrate-api-client/actions/runs/4664840889/jobs/8259110692

[bkchr]

CC @ggwpez

[ggwpez]

We should probably add that automated dependency license check to our CI as well, WDYT @bkchr?

[bkchr]

I think we once had one, not sure what happened to it.