Drop usage of `once_cell` in favor of std

[joshtriplett]

I think all the current uses of once_cell in gitoxide could be replaced by https://doc.rust-lang.org/std/cell/struct.LazyCell.html (stable since Rust 1.80), or by https://doc.rust-lang.org/std/cell/struct.OnceCell.html , or by https://doc.rust-lang.org/std/sync/struct.LazyLock.html if thread-safety is needed.

[EliahKagan]

It seems to me that this could be done now, since per #1518 (comment) the gitoxide MSRV usually tracks that of helix-editor, which was bumped to 1.82 about six months ago in helix-editor/helix#13275.

cc @Byron @spenserblack

[Byron]

Thanks for the hint, and thanks @EliahKagan for the reminder that we can now use a more recent compiler :).

It's notable that I was unable to find my notes on what we are actually tracking - I thought helix was effectively superseded by jiffy maybe? But I am probably wrong and generally would prefer to use more recent compilers.

[Byron]

Once get_or_try_init() is stabilized, we can complete the migration by updating the OnceCell type aliases to use std::sync::OnceLock and std::cell::OnceCell.

The first part has landed, but a couple of usages are left. With that, the MSRV was upped to Rustc 1.82.

For now blocked on rust-lang/rust#109737 (then the MSRV).