Update socket2 requirement from 0.3.19 to 0.4.0

Updates the requirements on [socket2](https://github.com/rust-lang/socket2) to permit the latest version.
- [Release notes](https://github.com/rust-lang/socket2/releases)
- [Changelog](https://github.com/rust-lang/socket2/blob/master/CHANGELOG.md)
- [Commits](rust-lang/socket2@v0.3.19...v0.3.19)

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

protocols/mdns/Cargo.toml

@@ -21,7 +21,7 @@ libp2p-swarm = { version = "0.28.0", path = "../../swarm" }
 log = "0.4.14"
 rand = "0.8.3"
 smallvec = "1.6.1"
-socket2 = { version = "0.3.19", features = ["reuseport"] }
+socket2 = { version = "0.4.0", features = ["reuseport"] }
 void = "1.0.2"
 
 [dev-dependencies]

transports/tcp/Cargo.toml

@@ -19,7 +19,7 @@ ipnet = "2.0.0"
 libc = "0.2.80"
 libp2p-core = { version = "0.27.0", path = "../../core" }
 log = "0.4.11"
-socket2 = { version = "0.3.17", features = ["reuseport"] }
+socket2 = { version = "0.4.0", features = ["reuseport"] }
 tokio-crate = { package = "tokio", version = "1.0.1", default-features = false, features = ["net"], optional = true }
 
 [features]