GH Actions: ubuntu-16.04 is no longer supported
GH Actions: ubuntu-16.04 is no longer supported

... use `ubuntu-18.04` or `ubuntu-latest` for `20.04` instead.

Also see:
* https://ubuntu.com/blog/ubuntu-16-04-lts-transitions-to-extended-security-maintenance-esm
* shivammathur/setup-php#452

paragonie/random_compat,paragonie/random_compat