Refactor signature extraction to make use of gix's .signature()

[Shnatsel]

gix 0.52 has added native support for extracting signatures in Byron/gitoxide#973

We should refactor this code to make use of the upstream function:

rustsec/rustsec/src/repository/git/commit.rs

Lines 78 to 93 in 7c05b2e

	// Note this is inefficient as gix doesn't yet support signature extraction natively.
	// TODO: convert this to native methods once https://github.com/Byron/gitoxide/pull/973 ships in a stable release.
	let signed_data = {
	let mut commit_without_signature = cref.clone();
	let pos = commit_without_signature
	.extra_headers
	.iter()
	.position(|eh| eh.0 == "gpgsig")
	.unwrap();
	commit_without_signature.extra_headers.remove(pos);
	let mut signed_data = Vec::new();
	use gix::objs::WriteTo;
	commit_without_signature.write_to(&mut signed_data)?;
	signed_data
	};