Update num-bigint requirement from 0.2 to 0.4

Updates the requirements on [num-bigint](https://github.com/rust-num/num-bigint) to permit the latest version.
- [Release notes](https://github.com/rust-num/num-bigint/releases)
- [Changelog](https://github.com/rust-num/num-bigint/blob/master/RELEASES.md)
- [Commits](rust-num/num-bigint@num-bigint-0.2.0...num-bigint-0.4.0)

Signed-off-by: dependabot[bot] <support@github.com>

Cargo.toml

@@ -22,7 +22,7 @@ web-view = { version = "0.7.2", features = [] }
 tinyfiledialogs = "3.3.10"
 serde = { version = "1.0.123", features = ["derive"] }
 serde_json = "1.0.42"
-num-bigint = "0.2"
+num-bigint = "0.4"
 num-traits = "0.2"
 bincode = "1.2.0"
 base64 = "0.11.0"