Update loom requirement from 0.5 to 0.6

Updates the requirements on [loom](https://github.com/tokio-rs/loom) to permit the latest version.
- [Release notes](https://github.com/tokio-rs/loom/releases)
- [Changelog](https://github.com/tokio-rs/loom/blob/master/CHANGELOG.md)
- [Commits](https://github.com/tokio-rs/loom/compare/v0.5.0...v0.6.0)

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

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

Cargo.toml

@@ -28,7 +28,7 @@ portable-atomic = { version = "1", default-features = false, optional = true }
 # Enables loom testing. This feature is permanently unstable and the API may
 # change at any time.
 [target.'cfg(loom)'.dependencies]
-loom = { version = "0.5", optional = true }
+loom = { version = "0.6", optional = true }
 
 [[bench]]
 name = "bench"