Release 0.21.3

[casey]

• Bump version: 0.21.2 → 0.21.3
• Update changelog
• Update changelog contributor credits
• Update dependencies

[casey]

Tried to pin rust-bitcoin to 0.32.3, but it fails because bip322 requires 0.32.4.

[raphjaph]

Downgrade bip322 to 0.0.8

[casey]

Downgraded bip322 to 0.8, and rebased onto latest master to pick up the palindrome charm.