Update core2 requirement from 0.3.0 to 0.4.0 in /lightning

Updates the requirements on [core2](https://github.com/bbqsrc/core2) to permit the latest version.
- [Release notes](https://github.com/bbqsrc/core2/releases)
- [Commits](https://github.com/bbqsrc/core2/commits)

---
updated-dependencies:
- dependency-name: core2
  dependency-type: direct:production
...

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

Author: dependabot[bot]

lightning/Cargo.toml

@@ -44,7 +44,7 @@ hex = { version = "0.4", optional = true }
 regex = { version = "0.2.11", optional = true }
 backtrace = { version = "0.3", optional = true }
 
-core2 = { version = "0.3.0", optional = true, default-features = false }
+core2 = { version = "0.4.0", optional = true, default-features = false }
 
 [dev-dependencies]
 hex = "0.4"