Updating `syn` to v3 in `zerocopy-derive`

Following the [recent release of `syn` v3](https://github.com/dtolnay/syn/releases/tag/3.0.0), would it be possible to update it in `zerocopy-derive` (and `prettyplease` to v0.3), with the aim of reducing dependency duplication in dependents? (I would PR it but I don't want to sign the CLA.)

Locate the dependency declarations for `zerocopy-derive` and `prettyplease`, then inspect the surrounding workspace configuration and existing dependency-update checks. Update `syn` to v3 and `prettyplease` to v0.3, and run the repository's relevant checks to confirm the derive crate and its dependents still build and test successfully.

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