fix daily builds errors. downgrade gradle openapi generator plugin.

[mshima]

gradle openapi generator v7.6.0 is correctly working with Gatling only.
downgrade to v7.5.0.

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Please make sure the below checklist is followed for Pull Requests.

• All continuous integration tests are green
• Tests are added where necessary
• The JDL part is updated if necessary
• jhipster-online is updated if necessary
• Documentation is added/updated where necessary
• Coding Rules & Commit Guidelines as per our CONTRIBUTING.md document are followed

When you are still working on the PR, consider converting it to Draft (below reviewers) and adding skip-ci label, you can still see CI build result at your branch.

[mshima]

#26203 was merged same day jdl daily-builds changes was merged 😒.
I was looking for a generation error, but was a dependency error.

[mshima]

@mshima I see that we were not using gradle plugin... Did you test with gradle plugin 7.6.0?

@DanielFran we are using gradle plugin, we were reusing maven-plugin version since it matches.
I've added a specific version for gradle plugin now.

So maven is v7.6.0 and gradle is been downgrade to v7.5.0

[mshima]

For reference: OpenAPITools/openapi-generator#18753