Update rand requirement from 0.7 to 0.8

Updates the requirements on [rand](https://github.com/rust-random/rand) to permit the latest version.
- [Release notes](https://github.com/rust-random/rand/releases)
- [Changelog](https://github.com/rust-random/rand/blob/master/CHANGELOG.md)
- [Commits](rust-random/rand@0.7.0...0.8.3)

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

Cargo.toml

@@ -36,7 +36,7 @@ strict_encoding = { version = "1.2", optional = true }
 subtle = { version = "2", default-features = false }
 hash_edwards_to_edwards = { git = "https://github.com/comit-network/hash_edwards_to_edwards" }
 integer-encoding = "3"
-rand = { version = "0.7", optional = true }
+rand = { version = "0.8", optional = true }
 lazy_static = "1"
 clear_on_drop = { version = "0.2", default-features = false }
 
@@ -45,7 +45,7 @@ version = "0.7.0"
 default-features = false
 
 [dev-dependencies]
-rand = { version = "0.7", features = ["std"] }
+rand = { version = "0.8", features = ["std"] }
 serde_json = "1"
 
 [package.metadata.docs.rs]