chore: update README rust version badge to 1.75.0

Author: notmandatory

README.md

@@ -13,7 +13,7 @@
     <a href="https://github.com/bitcoindevkit/bdk-cli/actions?query=workflow%3ACI"><img alt="CI Status" src="https://github.com/bitcoindevkit/bdk-cli/workflows/CI/badge.svg"></a>
     <a href="https://codecov.io/gh/bitcoindevkit/bdk-cli"><img src="https://codecov.io/gh/bitcoindevkit/bdk-cli/branch/master/graph/badge.svg"/></a>
     <a href="https://docs.rs/bdk-cli"><img alt="API Docs" src="https://img.shields.io/badge/docs.rs-bdk_cli-green"/></a>
-    <a href="https://blog.rust-lang.org/2021/12/02/Rust-1.57.0.html"><img alt="Rustc Version 1.57.0+" src="https://img.shields.io/badge/rustc-1.57.0%2B-lightgrey.svg"/></a>
+    <a href="https://blog.rust-lang.org/2023/12/28/Rust-1.75.0/"><img alt="Rustc Version 1.75.0+" src="https://img.shields.io/badge/rustc-1.75.0%2B-lightgrey.svg"/></a>
     <a href="https://discord.gg/d7NkDKm"><img alt="Chat on Discord" src="https://img.shields.io/discord/753336465005608961?logo=discord"></a>
   </p>