Modernize distro support and remove deprecated stdlib functions

[colonelpanik]

This removes the deprecated stdlib validate_ functions and adds Centos/RHEL/Rocky 8 support. Lightly tested on Ubuntu 20 as well and worked as-is with puppet 6 prior to this change so this should work there as well.

As the spec tests were relatively normal, I also took the chance to convert this to puppet pdk style module for easier unit testing. I can split these two changes off into separate PRs if you prefer.

[arcadius]

I am seeing the deprecations below @jonmosco
It would be great if we could have a new release with this pull request please.

Warning: This method is deprecated, please use the stdlib validate_legacy function, with Stdlib::Compat::String. There is further documentation for validate_legacy function in the README. at ["/etc/puppet/code/modules/logwatch/manifests/init.pp", 28]:["/tmp/mnc/puppet/manifests/site.pp", 0] (location: /etc/puppet/code/modules/stdlib/lib/puppet/functions/deprecation.rb:34:in deprecation')
Warning: This method is deprecated, please use the stdlib validate_legacy function,
with Stdlib::Compat::Array. There is further documentation for validate_legacy function in the README. at ["/etc/puppet/code/modules/logwatch/manifests/init.pp", 30]:["/tmp/mnc/puppet/manifests/site.pp", 0]
(location: /etc/puppet/code/modules/stdlib/lib/puppet/functions/deprecation.rb:34:in deprecation') Warning: This method is deprecated, please use the stdlib validate_legacy function, with Pattern[]. There is further documentation for validate_legacy function in the README. at ["/etc/puppet/code/modules/logwatch/manifests/init.pp", 32]:["/tmp/mnc/puppet/manifests/site.pp", 0] (location: /etc/puppet/code/modules/stdlib/lib/puppet/functions/deprecation.rb:34:in deprecation')
`