git_backend: update gix, adapt to breaking changes

Update `gix` to 0.72.1, and adapt to its breaking changes.

1. The signature of `gix::reference::iter::Platform::prefixed` changed
   in a way that seems to confuse Rust compiler (and does confuse me).

2. `git_object::Tree::EntryMode` API changed; `entry.mode()` now has a
   `value()` method.

3. Most significantly, the meaning and API of `gix::actor::SignatureRef`
   changed.

## Details about `gix::actor::SignatureRef`

The API for `gix::actor::Signature` and `gix::actor::SignatureRef`
changedd. The latter now contains an unparsed string time field, while
the former still contains a parsed time.  So, the conversions between
`gix::actor::SignatureRef` and either `gix::actor::Signature` or jj's
`Signature` types can now fail.

We use the epoch for the time if the timestamp is unreadable, like gix
did before.

Cc: GitoxideLabs/gitoxide#1935,
GitoxideLabs/gitoxide#2038

Author: ilyagr