(CONT-850) Puppet 8 support / Drop Puppet 6

[LukasAud]

This PR aims to update the module with compatibility for the upcoming
Puppet 8 agent release. The most relevant changes introduced in this
updated are:
Updated version requirements in metadata.json
Bumped Rubocop gems
Adjusted Rubocop configuration
Resolved new Rubocop offenses

[puppet-community-rangefinder]

accounts_ssh_authorized_keys_line_parser is a function

Breaking changes to this file MAY impact these 1 modules (near match):

• puppetlabs-accounts

accounts_ssh_options_parser is a function

Breaking changes to this file MAY impact these 1 modules (near match):

• puppetlabs-accounts

This module is declared in 3 of 580 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.