docs: update requirement

[sifnoc]

Tested at https://github.com/sifnoc/rust_witness_test/actions/runs/12904150297/job/35980761177#step:5:39

Checked version

+ rustup show
Default host: x86_64-unknown-linux-gnu
rustup home:  /home/runner/.rustup

installed toolchains
--------------------

stable-x86_64-unknown-linux-gnu (default)
nightly-20[24](https://github.com/sifnoc/rust_witness_test/actions/runs/12904150297/job/35980761177#step:5:25)-07-18-x86_64-unknown-linux-gnu
1.81.0-x86_64-unknown-linux-gnu

active toolchain
----------------

1.81.0-x86_64-unknown-linux-gnu (directory override for '/home/runner/work/rust_witness_test/rust_witness_test')
rustc 1.81.0 (eeb90cda1 2024-09-04)

In 1.84.0 installed, got error like at https://github.com/sifnoc/rust_witness_test/actions/runs/12904095695/job/35980604892#step:7:26

[chancehudson]

This error is not accurate. I ran the following (after removing rust-toolchain.toml):

➜  rust_witness_test git:(main) ✗ cargo -V
cargo 1.84.0 (66221abde 2024-11-19)
➜  rust_witness_test git:(main) ✗ cargo test
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.04s
     Running tests/main.rs (target/debug/deps/main-bf4998be2d2c8d5a)

running 1 test
test tests::test_multiplier2_witness ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

The error you're seeing is a build time linking error, I'm not sure of the exact cause though. I'd suggest removing the cache logic from the ci to start