Bump rand from 0.7.3 to 0.8.3 in /exonum-java-binding/core/rust

Bumps [rand](https://github.com/rust-random/rand) from 0.7.3 to 0.8.3.
- [Release notes](https://github.com/rust-random/rand/releases)
- [Changelog](https://github.com/rust-random/rand/blob/master/CHANGELOG.md)
- [Commits](https://github.com/rust-random/rand/commits)

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

Author: dependabot-preview[bot]

exonum-java-binding/core/rust/Cargo.lock

@@ -16,5 +16,5 @@ futures = "0.3"
 # Specify the "resource-manager" feature explicitly to prevent issues with linking (see ECR-2855)
 java_bindings = { path = "..", features = ["resource-manager"] }
 lazy_static = "1.3"
-rand = "0.7"
+rand = "0.8"
 serde = { version = "1.0", features = ["serde_derive"] }