Like #582, there is a new release of `signature` (v2.2.0) which contains
no breaking changes from ed25519-dalek's perspective. The main notable
one is it bumps MSRV to 1.60, which so also happens to also be
ed25519-dalek's MSRV.
This commit loosens the version requirement to allow `>=2.0, <2.3` to
allow the `signature` 2.2 series.
The `signature` crate contains unstable, minor version-gated
functionality.
The v2.1 release did not change any of that, and only added new
functionality. So it's safe to relax the requirement for `signature` to
`>=2.0, <2.2`.
