Remove further Qt support before Qt 5.15.2

[ghost]

This is a follow-up pull request for #19164, which removes the further handling of older Qt versions.

Ping @ravinikam @stkrwork @etherealjoy @muttleyxd

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh ./bin/configs/*.yaml
  ./bin/utils/export_docs_generators.sh
  

  (For Windows users, please run the script in Git BASH)
  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
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[wing328]

https://github.com/OpenAPITools/openapi-generator/actions/runs/11256311462/job/31329165228?pr=19824

please update the samples when you've time

[muttleyxd]

Looks good, but as wing328 mentioned - please update samples

[ghost]

Thx for the review. I will update the samples the next days.