Fix build in 1.83.0

Workaround for rust-lang/rust#133824

Author: andrewhickman

README.tpl

@@ -28,7 +28,7 @@ A protobuf library extending [`prost`](https://crates.io/crates/prost) with refl
 
 ## Minimum Supported Rust Version
 
-Rust **1.64** or higher.
+Rust **1.70** or higher.
 
 The minimum supported Rust version may be changed in the future, but it will be
 done with a minor version bump.