Update hex requirement from 0.3 to 0.4

Updates the requirements on [hex](https://github.com/KokaKiwi/rust-hex) to permit the latest version.
- [Release notes](https://github.com/KokaKiwi/rust-hex/releases)
- [Commits](KokaKiwi/rust-hex@v0.3...v0.4.3)

---
updated-dependencies:
- dependency-name: hex
  dependency-type: direct:production
...

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

Author: dependabot[bot]

lightning-invoice/Cargo.toml

@@ -24,4 +24,4 @@ core2 = { version = "0.3.0", default-features = false, optional = true }
 
 [dev-dependencies]
 lightning = { version = "0.0.104", path = "../lightning", default-features = false, features = ["_test_utils"] }
-hex = "0.3"
+hex = "0.4"

lightning/Cargo.toml

@@ -37,14 +37,14 @@ bitcoin = { version = "0.27", default-features = false, features = ["secp-recove
 secp256k1 = { version = "0.20.2", default-features = false, features = ["alloc"] }
 
 hashbrown = { version = "0.11", optional = true }
-hex = { version = "0.3", optional = true }
+hex = { version = "0.4", optional = true }
 regex = { version = "0.1.80", optional = true }
 backtrace = { version = "0.3", optional = true }
 
 core2 = { version = "0.3.0", optional = true, default-features = false }
 
 [dev-dependencies]
-hex = "0.3"
+hex = "0.4"
 regex = "0.1.80"
 # TODO remove this once rust-bitcoin PR #637 is released
 secp256k1 = { version = "0.20.2", default-features = false, features = ["alloc"] }