Force keyword-only args for Duration (prevent footgun)

[MadLittleMods]

Force keyword-only args for Duration (prevent footgun) so people have to specify which time unit they want to use.

Spawning from #19394 (comment)

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Code style is correct (run the linters)

[MadLittleMods]

When you forget the keyword arg now, the linter complains like this:

Duration(5)

synapse/handlers/worker_lock.py:264: error: Too many positional arguments for "Duration"  [misc]

[MadLittleMods]

Thanks for the review @erikjohnston 🦐