Upgrade Minitest to version 5.22.x (Drop Ruby 2.4 and 2.5 from CI)

[etagwerker]

• [x] Add an entry to CHANGELOG.md that links to this PR under the "main (unreleased)" heading.

Description:

It updates the minitest dependency to the latest stable version (5.22.x)

This means that we need to stop testing the library with Ruby 2.4 and 2.5, as minitest has dropped support for these versions.

I will abide by the code of conduct.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.02%. Comparing base (047e28d) to head (296ec99).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #115 +/- ## ======================================= Coverage 99.02% 99.02% ======================================= Files 17 17 Lines 307 307 ======================================= Hits 304 304 Misses 3 3 ```

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