build(deps): Update libsecp256k1 requirement from 0.5.0 to 0.6.0 (#2151)

Updates the requirements on [libsecp256k1](https://github.com/paritytech/libsecp256k1) to permit the latest version.
- [Release notes](https://github.com/paritytech/libsecp256k1/releases)
- [Changelog](https://github.com/paritytech/libsecp256k1/blob/master/CHANGELOG.md)
- [Commits](https://github.com/paritytech/libsecp256k1/commits)

---
updated-dependencies:
- dependency-name: libsecp256k1
  dependency-type: direct:production
...

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

core/Cargo.toml

@@ -18,7 +18,7 @@ fnv = "1.0"
 futures = { version = "0.3.1", features = ["executor", "thread-pool"] }
 futures-timer = "3"
 lazy_static = "1.2"
-libsecp256k1 = { version = "0.5.0", optional = true }
+libsecp256k1 = { version = "0.6.0", optional = true }
 log = "0.4"
 multiaddr = { version = "0.13.0" }
 multihash = { version = "0.14", default-features = false, features = ["std", "multihash-impl", "identity", "sha2"] }