feat: Switch to Rust edition 2024

Use resolver 3 (which came with Rust edition 2024).
Set the minimum required Rust version to 1.85.0.

Signed-off-by: David Runge <dvzrv@archlinux.org>

Cargo.toml

 [workspace]
-resolver = "2"
+resolver = "3"
 members = [
   "nethsm",
   "nethsm-backup",
@@ -44,10 +44,11 @@ authors = [
   "David Runge <dvzrv@archlinux.org>",
   "Wiktor Kwapisiewicz <wiktor@archlinux.org>",
 ]
-edition = "2021"
+edition = "2024"
 homepage = "https://gitlab.archlinux.org/archlinux/signstar"
 license = "Apache-2.0 OR MIT"
 repository = "https://gitlab.archlinux.org/archlinux/signstar"
+rust-version = "1.85.0"
 
 [profile.release]
 lto = true