PDK release with Platform 7 support

The current release of the PDK should support the current release of PE. This means that we need to get a Platform 7 supporting release out by PE 2021 release.

See related GitHub issues:

• https://github.com/puppetlabs/pdk/issues/931
• https://github.com/puppetlabs/pdk/issues/914
• https://github.com/puppetlabs/pdk/issues/889
• https://github.com/puppetlabs/pdk/issues/759
• https://github.com/puppetlabs/pdk/issues/885 - not critical to the 2.0 release
• https://github.com/ddfreyne/cri/pull/112#issuecomment-752775900 - the actual PDK command is always running on the latest ruby we ship, so this is only relevant for folks who want to run unsupported gem-based installations on outdated rubies.
• https://github.com/puppetlabs/pdk-templates/pull/392

Additional work needed:

• add PDK builds to puppet7 and puppet7-nightly repositories
• unbreak pdk-nightly shipping (last build available for Debian is 1.19.0.0.pre2.47.g618ff49-1buster from Nov 24)
  • unbreak pipelines on jenkins after master->main renaming