deps: update rust crate syn to v2.0.68

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
syn	dependencies	patch	2.0.67 -> 2.0.68

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Release Notes

dtolnay/syn (syn)

### [`v2.0.68`](https://togithub.com/dtolnay/syn/releases/tag/2.0.68) [Compare Source](https://togithub.com/dtolnay/syn/compare/2.0.67...2.0.68) - Improve panic location when `parse_quote!` parses invalid syntax ([#​1690](https://togithub.com/dtolnay/syn/issues/1690), thanks [@​stepancheg](https://togithub.com/stepancheg)) - More efficient peek implementation for `Group` and `Lifetime` ([#​1687](https://togithub.com/dtolnay/syn/issues/1687))

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