build: Update package version in pubspec.yaml to 2.2.0 and add to CHANGELOG.md

Author: gkc

CHANGELOG.md

@@ -1,3 +1,7 @@
+## 2.2.0
+- Update `asn1lib` dependency to ^1.5.0 and fix the resulting warnings about 
+  unnecessary null-safety operations in RSAPublicKey and RSAPrivateKey
+
 ## 2.1.0
 - Add `KeypairFactory`, `RSAKeypairFactory` and `ECKeypairFactory`
 - Update dependencies

pubspec.yaml

@@ -1,6 +1,6 @@
 name: crypton
 description: A simple Dart library for asymmetric encryption and digital signatures
-version: 2.1.0
+version: 2.2.0
 homepage: https://github.com/konstantinullrich
 repository: https://github.com/konstantinullrich/crypton
 issue_tracker: https://github.com/konstantinullrich/crypton/issues