chore(eslint): upgrade to v9 and fix resulting lint errors / issues

[ahennr]

Description

Related issues or pull requests

Pull request type

• Bugfix
• Feature
• Dependency updates
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe)

Do you introduce a breaking change?

• Yes
• No

Checklist

• I understand and agree that the changes in this PR will be licensed under the
  BSD-2-Clause.
• I have followed the guidelines for contributing.
• The proposed change fits to the content of the Code of Conduct.
• I have added or updated tests and documentation, and the test suite passes (run npm run check locally).
• I have added a screenshot/screencast to illustrate the visual output of my update.