(maint) Drop puppet 6 to 7 upgrade test

The puppet6 deb release packages contain a gpg keyring with a key that expired
225-04-06. If the release package is installed, then future apt-get update
command will report errors:

```
$ wget https://apt.puppet.com/puppet6-release-bullseye.deb
$ dpkg-deb -c puppet6-release-bullseye.deb | grep keyring
-rw-r--r-- root/root     10352 2022-12-09 23:52
./etc/apt/trusted.gpg.d/puppet6-keyring.gpg
$ sudo dpkg -i puppet6-release-bullseye.deb
$ sudo apt-get update
Hit:1 http://deb.debian.org/debian bullseye InRelease
Get:2 http://apt.puppetlabs.com bullseye InRelease [83.8 kB]
Err:2 http://apt.puppetlabs.com bullseye InRelease
  The following signatures were invalid: EXPKEYSIG 4528B6CD9E61EF26 Puppet, Inc. Release Key (Puppet, Inc. Release Key) <[email protected]>
```

Since puppet6 has been EOL for many years and we're not going to reissue
puppet6 release packages, drop puppet 6 to puppet 7 upgrade tests in the
puppet_agent module.

Author: joshcooper

acceptance/Rakefile

@@ -54,8 +54,6 @@ task :ci do
   begin
     Rake.application['prepare'].invoke
     case ENV['MASTER_COLLECTION']
-    when /puppet7/
-      beaker('exec ./tests/test_upgrade_puppet6_to_puppet7.rb')
     when /puppet8/
       beaker('exec ./tests/test_upgrade_puppet7_to_puppet8.rb')
     end