#25415 ci: build JAX-RS RESTEasy samples with Maven wrapper (3.8.8)

resolve build issues faced in https://github.com/OpenAPITools/openapi-generator/pull/25383. The tasks fails with:

Failed to execute goal org.apache.maven.plugins:maven-surefire-plugin:3.6.0:test (default-test) on project openapi-jaxrs-resteasy-server: TestNG support requires version 6.14.3 or above. You have declared version 6.8.8 -> [Help 1]

The RESTEasy samples declare TestNG 6.8.8 and don't pin
maven-surefire-plugin. On the runner's Maven 3.10 they default to
Surefire 3.6.0, which requires TestNG 6.14.3 or newer, so the test
phase fails.

Run these samples with ./mvnw (Maven 3.8.8), like the
microprofile-openapi-annotations sample. Also run the workflow when
the workflow file itself changes.

PR checklist

• Read the contribution guidelines.
• Run the following to build the project and update samples:

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

  (For Windows users, please run the script in WSL)
  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.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.