ci: read the MSRV from clippy.toml to avoid having too many copies to keep in sync

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/38807>

meson.build

@@ -788,9 +788,7 @@ if with_gallium_rusticl or with_nouveau_vk or with_tools.contains('etnaviv') or
   rustc = meson.get_compiler('rust')
   rust = import('rust')
 
-  # Keep this in sync with the MINIMUM_SUPPORTED_RUST_VERSION in
-  # .gitlab-ci/container/build-rust.sh and the `rustup default` line in
-  # .gitlab-ci/meson/build.sh and the `msrv` in clippy.toml
+  # Keep this in sync with the `msrv` in clippy.toml
   if rustc.version().version_compare('< 1.82')
     error('Mesa requires Rust 1.82.0 or newer')
   endif