Drop Ruby 2.5/2.6 support

[bastelfreak]

Both ruby version are EoL. We already merged a breaking change to master, so I think it's good to drop those old ruby versions.

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (f5071e1) 89.98% compared to head (c628225) 89.98%.

:exclamation: Current head c628225 differs from pull request most recent head a70a226. Consider uploading reports for the commit a70a226 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #478 +/- ## ======================================= Coverage 89.98% 89.98% ======================================= Files 76 76 Lines 1577 1577 ======================================= Hits 1419 1419 Misses 158 158 ``` [see 1 file with indirect coverage changes](https://codecov.io/gh/voxpupuli/json-schema/pull/478/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=voxpupuli) Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=voxpupuli). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=voxpupuli)

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[ekohl]

metadata-json-lint pulls this in, so it can end up being painful for a lot of users. IIRC bundler in Ruby 2.5 isn't able to deal well with the required ruby version. Take that into consideration.

[bastelfreak]

@ekohl metadata-json-lint uses json-schema '>= 2.8', '< 4.0'. The next major release will be 4.0 so metadata-json-lint would not automatically pull it in. I suggest we bump Ruby here and when we bump it in metadata-json-lint to 2.7 we can pull in metadata-json-lint 4.x.

[ekohl]

My concern is that we will want to update https://github.com/voxpupuli/metadata-json-lint/blob/e089948daf5f18db6a873f6da4d661858905968b/metadata-json-lint.gemspec#L20 but if we make it < 5 then Bundler on Ruby 2.5 probably won't be able to resolve it (see what happened recently with Bundler). That implicitly breaks Puppet 6.

IMHO :-1: on dropping it in this gem unless we start using Puppet 2.7 features or Puppet 6 is really dead and buried.