update MSRV shields.io badge (#5540)

Not sure this badge really has value, but it should be accurate 😄

Author: reaperhulk

README.md

@@ -4,7 +4,7 @@
 [![benchmark](https://img.shields.io/endpoint?url=https://codspeed.io/badge.json)](https://codspeed.io/PyO3/pyo3)
 [![codecov](https://img.shields.io/codecov/c/gh/PyO3/pyo3?logo=codecov)](https://codecov.io/gh/PyO3/pyo3)
 [![crates.io](https://img.shields.io/crates/v/pyo3?logo=rust)](https://crates.io/crates/pyo3)
-[![minimum rustc 1.63](https://img.shields.io/badge/rustc-1.63+-blue?logo=rust)](https://rust-lang.github.io/rfcs/2495-min-rust-version.html)
+[![minimum rustc 1.74](https://img.shields.io/badge/rustc-1.74+-blue?logo=rust)](https://rust-lang.github.io/rfcs/2495-min-rust-version.html)
 [![discord server](https://img.shields.io/discord/1209263839632424990?logo=discord)](https://discord.gg/33kcChzH7f)
 [![contributing notes](https://img.shields.io/badge/contribute-on%20github-Green?logo=github)](https://github.com/PyO3/pyo3/blob/main/Contributing.md)