build(rust): Update Rust toolchain to 1.91 (#2903)

Update Rust toolchain to 1.91 (1.91.0).

  **Changes:**
  - Update `rust-toolchain.toml` channel to `1.91`

**Release Notes:** https://github.com/rust-lang/rust/releases/tag/1.91.0

  ---
🤖 *This PR was created automatically by the Rust toolchain update bot.*

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

Author: github-actions[bot]

rust-toolchain.toml

@@ -1,4 +1,4 @@
 [toolchain]
 # We pin the minor version to prevent new Clippy lints from breaking CI.
 # But, we still want to pick up new patch versions.
-channel = "1.90"
+channel = "1.91"