(fix) Bump syntex_syntax version

Fixes the cascade of dependency breakages introduced in
rust-lang/rust#29850

Author: mcasper

dotenv_codegen/Cargo.toml

@@ -21,7 +21,7 @@ syntex = { version = "^0.22.0", optional = true }
 
 [dependencies]
 syntex = { version = "^0.22.0", optional = true }
-syntex_syntax = { version = "^0.22.0", optional = true }
+syntex_syntax = { version = "^0.23.0", optional = true }
 dotenv = "^0.7.0"
 
 [features]