(MODULES-11394) Remove Puppet 5 to 6 upgrade test

Puppet 5 went end-of-life in January 2021:

https://www.puppet.com/docs/puppet/5.5/about_agent.html

And Puppet 6 went end-of-life in February 2023:

https://www.puppet.com/docs/puppet/6/platform_lifecycle.html

Since both of these streams are now end-of-life, this commit
removes the acceptance test upgrading from Puppet 5 to Puppet 6.

Author: mhashizume