Bump syn from 2.0.66 to 2.0.68

Bumps [syn](https://github.com/dtolnay/syn) from 2.0.66 to 2.0.68.
- [Release notes](https://github.com/dtolnay/syn/releases)
- [Commits](dtolnay/syn@2.0.66...2.0.68)

---
updated-dependencies:
- dependency-name: syn
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

xnat_derive/Cargo.toml

@@ -16,5 +16,5 @@ anyhow = "1.0.86"
 attribute-derive = { version = "0.9.1", features = ["syn-full"] }
 proc-macro2 = "1.0.85"
 quote = "1.0.36"
-syn = { version = "2.0.66", features = ["full", "extra-traits"] }
+syn = { version = "2.0.68", features = ["full", "extra-traits"] }
 thiserror = "1.0.60"