Reimplement Set as Ds\Set in CertificateRetrievingAndCreation.php

[dxL1nus]

Improvement suggestion made by @nuest in #49 (comment)

Starting point: https://www.php.net/manual/en/class.ds-set.php

---

 9   $dotenv = Dotenv::createImmutable(__DIR__);
10   $dotenv->load();
11
12   class Set

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References: https://github.com/codecheckers/ojs-codecheck/blob/main/CertificateRetrievingAndCreation.php

[dxL1nus]

@nuest I am now waiting on the Pull Request #56 to be merged, so that this issue can be closed.