Update syn requirement from ^1 to ^2

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...2.0.50)

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

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

macros/Cargo.toml

@@ -14,4 +14,4 @@ proc-macro = true
 
 [dependencies]
 quote = "1.0.3"
-syn = { version = "^1", features = ["full"] }
+syn = { version = "^2", features = ["full"] }