Update syn requirement from 1.0 to 2.0 (#611)

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...2.0.2)

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

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

gotham_derive/Cargo.toml

@@ -15,7 +15,7 @@ keywords = ["http", "async", "web", "framework", "gotham"]
 edition = "2018"
 
 [dependencies]
-syn = "1.0"
+syn = "2.0"
 quote = "1.0"
 
 [lib]