build: bump minimum supported rust version to 1.85

Update the rust-version in Cargo.toml and the installation requirements in README.md to reflect the new MSRV.

Cargo.toml

@@ -10,7 +10,7 @@ resolver = "2"
 [workspace.package]
 edition = "2021"
 license = "Apache-2.0"
-rust-version = "1.75"
+rust-version = "1.85"
 repository = "https://github.com/Lochnair/irtt-rs"
 homepage = "https://github.com/Lochnair/irtt-rs"
 readme = "README.md"

README.md

@@ -24,7 +24,7 @@ confusing overlap with upstream IRTT.
 
 ## Install
 
-Requires Rust 1.75 or newer.
+Requires Rust 1.85 or newer.
 
 From a local checkout: