Add an MSRV, Rust 1.74

This version is needed for `std::io::Error::other(_)`; I verified that
everything else still works on Rust 1.74 as well.

Author: musicinmybrain

Cargo.toml

@@ -7,6 +7,7 @@ description = "A library for streaming reading of files over HTTP using range re
 license = "MIT"
 repository = "https://github.com/prefix-dev/async_http_range_reader"
 exclude = ["test-data/*"]
+rust-version = "1.74"
 
 [dependencies]
 futures = "0.3.28"