build: update syn to version 3.0

Cargo.toml

@@ -25,7 +25,7 @@ doctest = false
 [dependencies]
 proc-macro2 = "1.0"
 quote = "1.0"
-syn = { version = "2.0", features = ["full"] }
+syn = { version = "3.0", features = ["full"] }
 
 [dev-dependencies]
 serde = { version = "1.0", features = ["derive"] }