Bump pointycastle from 3.9.1 to 4.0.0

Bumps [pointycastle](https://github.com/bcgit/pc-dart) from 3.9.1 to 4.0.0.
- [Changelog](https://github.com/bcgit/pc-dart/blob/master/CHANGELOG.md)
- [Commits](https://github.com/bcgit/pc-dart/commits)

---
updated-dependencies:
- dependency-name: pointycastle
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

pubspec.yaml

@@ -10,7 +10,7 @@ dependencies:
   asn1lib: ^1.5.8
   convert: ^3.1.2
   meta: ^1.16.0
-  pointycastle: ^3.9.1
+  pointycastle: ">=3.9.1 <5.0.0"
   pinenacl: ^0.6.0
 
 dev_dependencies: