Fixed gpg file for Ubuntu versions 21.04 and later.

[Conzar]

As of Ubuntu 21.04, Canonical has again changed the filename of the gpg files going back to dashes
instead of using underscores.

I have tested this change for Ubuntu 21.04 and 21.10. I also checked Ubuntu 20.04 just to make sure it didn't break previous versions.

[CLAassistant]

All committers have signed the CLA.

[puppet-community-rangefinder]

apt::ppa is a type

that may have no external impact to Forge modules.

This module is declared in 233 of 578 indexed public Puppetfiles.

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These results were generated with Rangefinder, a tool that helps predict the downstream impact of breaking changes to elements used in Puppet modules. You can run this on the command line to get a full report.

Exact matches are those that we can positively identify via namespace and the declaring modules' metadata. Non-namespaced items, such as Puppet 3.x functions, will always be reported as near matches only.