Add Sendable conformance to Xoshiro and LCRNG generators

[seanmrich]

The current implementation of swift-gen supports back to swift 5.0. However, with the release of swift 6.0 the Sendability requirements have become more strict. An example for this project is using the generators with the withRandomNumberGenerator dependency in the swift-dependencies package. Using e.g. Xoshiro raises an error:

withDependencies {
  $0.withRandomNumberGenerator = WithRandomNumberGenerator(Xoshiro(seed: 0))
// Type 'Xoshiro' does not conform to the 'Sendable' protocol 👆
}

All that's required is the declaration for the included generators to support sendability.

[stephencelis]

Thanks! This library could definitely use some updates, so we appreciate the time!

[stephencelis]

@seanmrich Do you have time to look into updating the Linux CI to point to a more recent Swift to get this PR green? (If not, no worries, we can take a look!)

[seanmrich]

Looks like the error was related to the Docker image using swift 5.3, but my PR increased the requirement to swift 5.5 to get Sendable. Added a commit to restore the tools version to 5.0 and instead put the Sendable conformance behind a swift version check.

Note that the conformance added on LCRNG creates a warning because the type is deprecated. Not sure if you want to remove conformance since it's deprecated anyway just to remove the warning.