Use PKey.read when loading private keys

There's a littany of reasons that we couldn't use PKey.read in https://github.com/puppetlabs/puppet/blob/1a13e0cf96c70b303492e684f9ccf4c38207b3dd/lib/puppet/x509/cert_provider.rb#L218-L222.

However, We no longer use this code in Terminii that will be loaded in JRuby (and so don't use this code at all in JRuby) nor do we support older versions of Ruby in Puppet 7.x. Our manual determination of which implementation class to construct is somewhat naive and PKey.read will do a better job.

Consequently, we should use PKey.read in the above code.

Note: This should only be updated for puppet7 (main branch).