Updates for experimental MSC4388 support (sign-in with QR code)

[hughns]

Changes:

• Renamed the msc4388_mode config option from public to open which is a more accurate name
• Updates the discovery mechanism to match the latest from MSC4388 (removed the unstable_features value from /versions response and adds the dedicated discovery endpoint)

Experimental feature tracking issue: #19433

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 (!).
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• Code style is correct (run the linters)

[hughns]

Sorry, but there is one more change to include in this which I have just pushed. Thank you.