Drop Ruby 2.4/2.5/2.6 support; Implement RuboCop

[bastelfreak]

Also contains #110

[codecov[bot]]

Codecov Report

Patch coverage: 42.85% and project coverage change: -1.38 :warning:

Comparison is base (c956a2c) 49.46% compared to head (da98534) 48.09%.

:exclamation: Current head da98534 differs from pull request most recent head c972900. Consider uploading reports for the commit c972900 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #109 +/- ## ========================================== - Coverage 49.46% 48.09% -1.38% ========================================== Files 8 8 Lines 471 472 +1 ========================================== - Hits 233 227 -6 - Misses 238 245 +7 ``` | [Impacted Files](https://app.codecov.io/gh/voxpupuli/puppet-blacksmith/pull/109?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=voxpupuli) | Coverage Δ | | |---|---|---| | [lib/puppet\_blacksmith/rake\_tasks.rb](https://app.codecov.io/gh/voxpupuli/puppet-blacksmith/pull/109?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=voxpupuli#diff-bGliL3B1cHBldF9ibGFja3NtaXRoL3Jha2VfdGFza3MucmI=) | `0.00% <0.00%> (ø)` | | | [lib/puppet\_blacksmith/version\_helper.rb](https://app.codecov.io/gh/voxpupuli/puppet-blacksmith/pull/109?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=voxpupuli#diff-bGliL3B1cHBldF9ibGFja3NtaXRoL3ZlcnNpb25faGVscGVyLnJi) | `45.36% <42.50%> (-1.04%)` | :arrow_down: | | [lib/puppet\_blacksmith/forge.rb](https://app.codecov.io/gh/voxpupuli/puppet-blacksmith/pull/109?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=voxpupuli#diff-bGliL3B1cHBldF9ibGFja3NtaXRoL2ZvcmdlLnJi) | `78.30% <66.66%> (-0.21%)` | :arrow_down: | | [lib/puppet\_blacksmith/git.rb](https://app.codecov.io/gh/voxpupuli/puppet-blacksmith/pull/109?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=voxpupuli#diff-bGliL3B1cHBldF9ibGFja3NtaXRoL2dpdC5yYg==) | `90.74% <78.94%> (-2.25%)` | :arrow_down: | | [lib/puppet\_blacksmith/modulefile.rb](https://app.codecov.io/gh/voxpupuli/puppet-blacksmith/pull/109?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=voxpupuli#diff-bGliL3B1cHBldF9ibGFja3NtaXRoL21vZHVsZWZpbGUucmI=) | `89.79% <88.88%> (ø)` | |

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.

[alexjfisher]

Do we have to be this aggressive? Whilst the latest rubocop still supports Ruby 2.5, do we gain anything by dropping more than just 2.4?

[bastelfreak]

We need to drop Ruby 2.5 in some of the beaker tooling in order to support ruby 3.2. So I reverse the question: what do we gain by not dropping Ruby 2.5 here? Our pipeline won't support Ruby 2.5 anyways. And the actual release is using Ruby 3.0 since a long long time. That's the only scenario where puppet-blacksmith is used.

And if we drop Ruby < 2.6 here we can use the same rubocop configuration as for beaker and other tools we have, and don't maintain multiple configs/CI matrixes.