build(deps): update syn requirement from 1.0 to 2.0

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@1.0.0...2.0.28)

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

Signed-off-by: dependabot[bot] <[email protected]>

gtk4-macros/Cargo.toml

@@ -26,7 +26,7 @@ proc-macro-crate = "1.0"
 proc-macro-error = "1.0"
 proc-macro2 = "1.0"
 quote = "1.0"
-syn = {version = "1.0", features = ["full"]}
+syn = {version = "2.0", features = ["full"]}
 
 [dev-dependencies]
 futures-channel = "0.3"