Bump rand from 0.7.3 to 0.8.6 in /app/rust

Bumps [rand](https://github.com/rust-random/rand) from 0.7.3 to 0.8.6.
- [Release notes](https://github.com/rust-random/rand/releases)
- [Changelog](https://github.com/rust-random/rand/blob/0.8.6/CHANGELOG.md)
- [Commits](rust-random/rand@0.7.3...0.8.6)

---
updated-dependencies:
- dependency-name: rand
  dependency-version: 0.8.6
  dependency-type: direct:production
...

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

Author: dependabot[bot]

app/rust/Cargo.lock

@@ -10,7 +10,7 @@ name = "rslib"
 crate-type = ["staticlib"]
 
 [dependencies]
-rand={ version = "0.7.3", default-features = false}
+rand={ version = "0.8.6", default-features = false}
 merlin = {version = "2.0.0", default-features=false}
 zeroize = {version = "1.1.1", default-features=false}