Bump rust-toolchain from 1.90 to 1.91

Bumps [rust-toolchain](https://github.com/rust-lang/rust) from 1.90 to 1.91.
- [Release notes](https://github.com/rust-lang/rust/releases)
- [Changelog](https://github.com/rust-lang/rust/blob/master/RELEASES.md)
- [Commits](rust-lang/rust@1.90.0...1.91.0)

---
updated-dependencies:
- dependency-name: rust-toolchain
  dependency-version: '1.91'
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

rust-toolchain.toml

@@ -1,5 +1,5 @@
 [toolchain]
-channel = "1.90"
+channel = "1.91"
 components = [
   "rustc",
   "cargo",