Update MSRV to 1.91

[ntBre]

Summary

• Bump the Rust toolchain to 1.93
• Bump our MSRV to 1.91

This latter change also allows us to drop our copy of ceil_char_boundary, which is now stable. I'm not 100% sure we want to do that, as the TextSize API seems nice, but we can if we want.

I quickly looked through the clippy release notes and didn't see exactly why this lint expectation is now unfulfilled, but that was the only new clippy diagnostic from the upgrade.

Note: I don't think we can land this until after we bump our conda-forge version in conda-forge/ruff-feedstock#318.

Test Plan

Existing tests

[astral-sh-bot]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

Formatter (stable)

✅ ecosystem check detected no format changes.

Formatter (preview)

✅ ecosystem check detected no format changes.

[dhruvmanila]

I'm fine with removing the custom implementation.

[dcreager]

Yesssssss! I've been waiting for std::iter::chain for ages!