Drop Ruby =< 2.4 support

[bastelfreak]

@ekohl we can also directly drop Ruby 2.4 and move to 2.5 or 2.7 if you like.

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (d749fb3) 50.23% compared to head (252540f) 50.23%.

:exclamation: Current head 252540f differs from pull request most recent head f87872f. Consider uploading reports for the commit f87872f to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #125 +/- ## ======================================= Coverage 50.23% 50.23% ======================================= Files 5 5 Lines 211 211 ======================================= Hits 106 106 Misses 105 105 ``` | [Impacted Files](https://codecov.io/gh/voxpupuli/metadata-json-lint/pull/125?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=voxpupuli) | Coverage Δ | | |---|---|---| | [lib/metadata-json-lint/schema.rb](https://codecov.io/gh/voxpupuli/metadata-json-lint/pull/125?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=voxpupuli#diff-bGliL21ldGFkYXRhLWpzb24tbGludC9zY2hlbWEucmI=) | `70.37% <ø> (ø)` | |

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[ekohl]

If you're dropping <= 2.3, I'd certainly also drop 2.4 now. I'd be tempted to keep 2.5 at least for a bit now and only drop it if you need to for Ruby 3.2 support.

[bastelfreak]

@ekohl This now drops Ruby 2.3 and Ruby 2.4