[ci] Roll pinned stable toolchain

[google-pr-creation-bot]

Automated changes by create-pull-request GitHub action

[gemini-code-assist]

Summary of Changes

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This pull request primarily focuses on maintaining the project's compatibility and improving developer experience by updating the stable Rust toolchain and refining compiler diagnostics. The changes ensure that the project builds with the latest stable Rust version and that error messages are more informative and actionable, particularly for common issues like missing trait implementations and incorrect transmute usage.

Highlights

• Rust Toolchain Update: The pinned stable Rust toolchain used in CI has been updated from version 1.90.0 to 1.91.1.
• Improved Compiler Diagnostics: Error messages for unsatisfied trait bounds across various UI tests have been enhanced. The diagnostics now provide more explicit 'help' notes, including the source location of the type that failed to implement the required trait.
• Enhanced Transmute Error Messages: The compiler now reports a more specific error code (E0080) for transmute! operations that involve size mismatches, preceding the general E0512 error, offering clearer feedback on the nature of the error.
• Minor Diagnostic Wording Updates: Notes for unused_imports and conflicting_repr_hints warnings have been slightly rephrased to indicate their inclusion within broader warning groups (e.g., #[warn(unused)] and #[deny(future_incompatible)]).

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[gemini-code-assist]

Code Review

This automated pull request updates the pinned stable Rust toolchain from 1.90.0 to 1.91.1. The changes primarily consist of updating UI test snapshots (.stderr files) to match the new diagnostic output from the updated compiler. These changes are expected and appear correct. No issues were found in this review.