CMake: Helpful error message when Rust version too old

[val-ms]

It isn't obvious if a build fails because the Rust version is too low.

The FindRust.cmake module already has logic to check the min supported Rus version. This commit make use of that, adding a Rust version var at the top of CMakeLists.txt. We will have to update it any time we run cargo update, before releasing a new feature version.

If your Rust version is too low (or you artificially crank up the min version for testing, the error will look like this:

-- Rust tool `cargo` found: /home/micah/.cargo/bin/cargo, 1.85.0 -- Rust tool `rustc` found: /home/micah/.cargo/bin/rustc, 1.85.0 -- Rust tool `rustup` found: /home/micah/.cargo/bin/rustup, 1.28.1 -- Rust tool `rust-gdb` found: /home/micah/.cargo/bin/rust-gdb, 9.2 -- Rust tool `rust-lldb` not found: Failed to determine version. -- Rust tool `rustdoc` found: /home/micah/.cargo/bin/rustdoc, 1.85.0 -- Rust tool `rustfmt` found: /home/micah/.cargo/bin/rustfmt, 1.8.0 -- Rust tool `bindgen` not found.
CMake Error at cmake/FindRust.cmake:412 (message):
  Your Rust toolchain is to old to build this project:

      1.85.0 < 1.100.0
Call Stack (most recent call first):
  CMakeLists.txt:144 (find_package)

-- Configuring incomplete, errors occurred!

Fixes: #1480

I have also updated the min Rust version in clamav-documentation where it talks about installing Rust: Cisco-Talos/clamav-documentation@02f6902

[val-ms]

This PR's new test hangs in the github runner and I have no idea why. Will mark it as a draft until I can revisit and figure it out.