Raise minimum version for syn-codegen crate to Rust 1.65

Required by hashbrown 0.15.1.

    error: package `hashbrown v0.15.1` cannot be built because it requires rustc 1.65.0 or newer, while the currently active rustc version is 1.63.0

Author: dtolnay

.github/workflows/ci.yml

@@ -42,7 +42,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        rust: [stable, beta, 1.63.0, 1.61.0]
+        rust: [stable, beta, 1.65.0, 1.61.0]
         include:
           - rust: nightly
             components: rustc-dev