Update upper bound of pointycastle (#65)

* Update upper bound of `pointycastle`

* Update versions

Author: Carapacik

pubspec.yaml

@@ -5,7 +5,7 @@ repository: https://github.com/jonasroussel/dart_jsonwebtoken
 homepage: https://github.com/jonasroussel/dart_jsonwebtoken#readme
 
 environment:
-  sdk: ">=2.12.0 <4.0.0"
+  sdk: ">=3.0.0 <4.0.0"
 
 false_secrets:
   - /example/*.pem
@@ -14,7 +14,7 @@ false_secrets:
 
 dependencies:
   crypto: ^3.0.6
-  pointycastle: ^3.7.3
+  pointycastle: ^4.0.0
   convert: ^3.1.2
   collection: ^1.17.1
   ed25519_edwards: ^0.3.1