Update Rust version to 1.74.1

We originally pinned to 1.63 because that was the last version before
Rust dropped support for older versions of `glibc` [in Rust 1.64][blog].
However, the relevant operating systems we were maintaining support for
at that time (CentOS/RHEL 6) have now been end-of-life for three full
years, so it seems reasonable to update to a more current version. Since
no other changes to the `glibc` support have happened in the interval,
we can safely update to the most current version of Rust.

[blog]: https://blog.rust-lang.org/2022/08/01/Increasing-glibc-kernel-requirements.html

rust-toolchain.toml

@@ -1,4 +1,4 @@
 [toolchain]
-channel = "1.63"
+channel = "1.74"
 components = ["clippy", "rustfmt"]
 profile = "minimal"