Update puppet_agent module to use new GPG key

Currently there are 2 GPG keys in the puppet_agent module:

• files/GPG-KEY-puppetlabs: this expired in 2017 and I think we're safe to remove it
• files/GPG-KEY-puppet: this one expires in 2021

Support should also be added for the new GPG key which expires in 2025: RPM-GPG-KEY-puppet-20250406
The new key can be found here: http://apt.puppet.com/DEB-GPG-KEY-puppet-20250406

Similar work was done in this PR: https://github.com/puppetlabs/puppetlabs-puppet_agent/pull/151

/cc morgan or eric.griswold: is the name for the new GPG key final? also please let me know if there's anything else I missed. Thanks!