Add kernel version check (#673) [ci skip]

Add kernel version check (#673) [ci skip]

On Ruby 3.1 there is a `RuntimeException` when calling `Random.urandom` on platforms where it's not properly implemented, which includes people running Ubuntu 14.04 with kernel 3.13.

See

https://meta.discourse.org/t/docker-build-broken-site-down/252047
https://meta.discourse.org/t/undefined-method-deprecated-for-nil-nilclass-on-rebuild/251711
https://meta.discourse.org/t/discourse-update-error/253197
https://meta.discourse.org/t/bootstrap-failure-during-most-recent-upgrade/253424