DOC: update README, BUG: typo in code

Summary

• Update maven openapi-plugin to 7.6.0
• Add Troubleshooting info for building the API under windows
• Fix "DefaultMetadat" typo

Test Environment

• Manual testing under windows.

Check List for the Author

Please, prepare your MR for a review. Be sure to write a summary and a focus and create gitlab comments for the reviewer. They should guide the reviewer through the changes, explain your changes and also point out open questions. For further good practices have a look at our review guidelines

• All automated tests pass
• Reference related issues
• Up-to-date CHANGELOG.md (or not necessary)
• Up-to-date JSON schema (or not necessary)
• Appropriate user and developer documentation (or not necessary)
  • Update / write published documentation (make doc).
  • How do I use the software? Assume "stupid" users.
  • How do I develop or debug the software? Assume novice developers.
• Annotations in code (Gitlab comments)
  • Intent of new code
  • Problems with old code
  • Why this implementation?

Check List for the Reviewer

• I understand the intent of this MR
• All automated tests pass
• Appropriate user and developer documentation (or not necessary), also in published documentation.
• The test environment setup works and the intended behavior is reproducible in the test environment
• In-code documentation and comments are up-to-date.

For further good practices have a look at our review guidelines.