Update rust.yml

Avoid testing on nightly and beta.

It fails there, but should pass on stable.

The error usually is

```
error[E0635]: unknown feature `proc_macro_span_shrink`
  --> /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.51/src/lib.rs:92:30
   |
92 |     feature(proc_macro_span, proc_macro_span_shrink)
   |                              ^^^^^^^^^^^^^^^^^^^^^^
```

For example https://github.com/ccouzens/vcloud-rest-openapi/actions/runs/7366189644/job/20048203866

.github/workflows/rust.yml

@@ -15,8 +15,6 @@ jobs:
       matrix:
         toolchain:
           - stable
-          - beta
-          - nightly
     steps:
       - uses: actions/checkout@v3
       - run: cd transformer && rustup update ${{ matrix.toolchain }} && rustup default ${{ matrix.toolchain }}