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...2.0.37)

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

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

robusta-codegen/Cargo.toml

@@ -15,7 +15,7 @@ proc-macro = true
 [dependencies]
 quote = "^1"
 proc-macro2 = { version = "1.0.21", features = ["span-locations"]}
-syn = { version = "^1", features = ["visit", "fold", "derive"] }
+syn = { version = "^2", features = ["visit", "fold", "derive"] }
 proc-macro-error = { version = "1", default-features = false }
 rand = "^0"
 darling = "^0.14"