cargo: set MSRV to 1.85

I believe the current stable version of Debian packages 1.85 rustc. So
if the next release of ripgrep uses a higher MSRV, then I think Debian
won't be able to package it.

It also turned out that I wasn't using anything from beyond Rust 1.85
anyway.

It's likely that I could make use of let-chains in various places, but I
don't think it's worth combing through the code to switch to them at
this point.

Cargo.toml

@@ -25,7 +25,7 @@ exclude = [
 build = "build.rs"
 autotests = false
 edition = "2024"
-rust-version = "1.88"
+rust-version = "1.85"
 
 [[bin]]
 bench = false