Infra: Upgrade openapi generator to 7.15.0

[yeikel]

What changes did you make? (Give an overview)

Upgrades the org.openapi.generator to the latest version (7.15.0)

Cherry-picked from #1366

How Has This Been Tested? (put an "x" (case-sensitive!) next to an item)

• Covered by existing automation

Checklist (put an "x" (case-sensitive!) next to all the items, otherwise the build will fail)

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (e.g. ENVIRONMENT VARIABLES)
• My changes generate no new warnings (e.g. Sonar is happy)
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged

Check out Contributing and Code of Conduct

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