Update README with proper version of Rust that is baseline
* [Docs](https://docs.rs/async-graphql)
* [GitHub repository](https://github.com/async-graphql/async-graphql)
* [Cargo package](https://crates.io/crates/async-graphql)
-* Minimum supported Rust version: 1.46 or later
+* Minimum supported Rust version: 1.49.0 or later
## Safety
