[Proposal] Upgrade to Rust 1.91

[kaimast]

💥 Proposal

Rust 1.91 was released over two weeks ago. It includes an upgrade of LLVM among other changes. The LLVM upgrade is particularly relevant as it could decrease our build times and generate more efficient binaries.

In terms of complexity, this change should be much smaller than #2788 as the latter moved from Rust 2021 to Rust 2024. There will be new clippy lints, though, which most likely can be addressed automatically, or we can disable some of them.

Rust 1.91 also enables linking with LLD by default, which we enabled in #2816.