build(deps): update syn requirement from 1.0.8 to 2.0.3

Updates the requirements on [syn](https://github.com/dtolnay/syn) to permit the latest version.
- [Release notes](https://github.com/dtolnay/syn/releases)
- [Commits](dtolnay/syn@1.0.8...2.0.3)

---
updated-dependencies:
- dependency-name: syn
  dependency-type: direct:production
...

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

varlink_generator/Cargo.toml

@@ -27,7 +27,7 @@ varlink_parser = { version = "4.2", path = "../varlink_parser" }
 quote = "1.0.2"
 proc-macro2 = "1.0.6"
 getopts = "0.2.21"
-syn = "1.0.8"
+syn = "2.0.3"
 chainerror = "0.7.0"
 
 [dev-dependencies]