Bump hex from 0.3.2 to 0.4.0 (#5930)

Bumps [hex](https://github.com/KokaKiwi/rust-hex) from 0.3.2 to 0.4.0.
- [Release notes](https://github.com/KokaKiwi/rust-hex/releases)
- [Commits](https://github.com/KokaKiwi/rust-hex/commits)

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

merkle-tree/Cargo.toml

@@ -12,7 +12,7 @@ edition = "2018"
 solana-sdk = { path = "../sdk", version = "0.19.0-pre0" }
 
 [dev-dependencies]
-hex = "0.3.2"
+hex = "0.4.0"
 
 [lib]
 crate-type = ["lib"]