CI fails to build against ruby 2.7 / 3.0 for puppet 7

CI fails to build against ruby 2.7 / 3.0 and puppet 7; and possibly may fail for ruby 2.7 / 3.0 and puppet 6.

This should be fixed, given that ruby 2.7 and 3.0 are now considered stable releases (they are not yet EOL, but we shouldn't wait that long if we can avoid it).

Start by inspecting the repository's CI configuration and reproducing the build for Ruby 2.7 and 3.0 with Puppet 7, then check Puppet 6 as indicated. Done means the affected CI builds pass for the supported Ruby and Puppet combinations.

索引モデルが issue の本文から書いたものです。