What does this PR do?

Adds spec tests for Ruby 2.7 and 3.0, as well as Chef 17.

This requires installing a newer version of rvm, because the default version present in the image doesn't know Ruby 3.0.

Updates the tests to be compatible with Ruby 3.0:

• the fauxhai gem had to be updated to 8.3.1 to include this fix: Resolve warning about usage of URI.open chefspec/fauxhai#351, without which the tests fail (because open alone can only refer to File.open, not URI.open in Ruby 3.0)
• the fauxhai gem update deprecated a bunch of platforms: Fedora 26 & 27, Debian 7.11 and 8.9. They have been replaced with Fedora 31 & 32, Debian 8.11.
• the Fedora platforms update required some changes in tests, as these versions use dnf_package and not yum_package.

Updates the tests & rake tasks to be compatible with Chef 17:

• in Chef 17, the zypper_repository resource automatically transforms the gpgkey entry into an array. The corresponding test had to be modified to take the Chef >= 17 and Chef < 17 cases into account.
• some chef modules used in release rake tasks have disappeared. As a temporary workaround, these are lazy-loaded so that they don't make Chef 17 CI tests fail.