Release tracking issue for `signature` 2.0-dependent crates

[tarcieri]

We're getting close to ready to cut a final release of signature v2.0.0: RustCrypto/traits#1171 Edit: released!

This is a tracking and discussion issue for any final changes before releasing the following:

• dsa v0.5.0 #619
• ecdsa v0.15.0 #620
• ed25519 v2.0.0 #622

[tarcieri]

FYI: the planned release date for signature v2.0.0 is one week from today: 2022-01-15

RustCrypto/traits#1171 (comment)

[tarcieri]

All crates in this repo now use signature v2