upgrade to gix@0.81.0 (#16828)

## Summary
- update `gix` from `0.80.0` to `0.81.0`
- add the explicit `sha1` feature now required when using `gix` with
`default-features = false`
- refresh the lockfile for the updated `gix` dependency family

Closes #16815

## Why
This updates Cargo to the latest published `gix` release while
preserving the existing feature set. The extra `sha1` feature keeps the
previous hash behavior intact after the `gix 0.81.0` feature layout
change.

## Validation
- `cargo check -p cargo`
- `cargo test -p cargo --no-run`
- `cargo check --workspace --all-targets`

## Notes
This change was driven by Codex at Byron's request so CI could run on
the updated dependency set. Byron still needs to review the patch before
anything is merged.

Author: weihanglo