update sample tests, fix Java tests

[martin-mfg]

• Running ./bin/generate-samples.sh does not update generated test files, because "Test files never overwrite an existing file of the same name." Therefore many test files in the samples directory were outdated. I temporarily changed the OpenAPI Generator code to allow overwriting test files and then ran ./bin/generate-samples.sh.
  (@wing328 As a permanent solution for this problem, we could introduce a new setting which allows test files to be overwritten. If you think this is a good idea, please let me know. I would then create a PR for this.)
• I replaced the outdated forkMode setting in Java generators. (more info on StackOverflow)
• I removed junit-platform-runner in those cases where it is not necessary anymore. If the junit-platform-runner dependency is present and we are using maven-surefire-plugin version 3, no tests are being run.
• I updated the non-generated, manually added test class samples/client/echo_api/java/resttemplate/src/test/java/org/openapitools/client/CustomTest.java from JUnit 4 to JUnit 5.

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

  (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.
  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.
• File the PR against the correct branch: master (upcoming 7.x.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.
  @bbdouglas (2017/07) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01) @karismann (2019/03) @Zomzog (2019/04) @lwlee2608 (2019/10) @martin-mfg (2023/08)