feat(capabilities): add sip-dialin-info capability

[ShGKme]

☑️ Resolves

• A part of: Inconsistency of the Initial State and Capabilities  #8715

🛠️ API Checklist

🚧 Tasks

• Add sip-dialin-info available only in the initial state to capabilities for clients
• Use capabilities instead of initial state on frontend
• Remove unneeded initial state

🏁 Checklist

• ⛑️ Tests (unit and/or integration) are included or not possible
• 📘 API documentation in docs/ has been updated or is not required
• 🔖 Capability is added or not needed

[nickvergessen]

I'm not sure why we went with initial state back then.
I think we should have this on a dedicated endpoint.

It actually is already. Could we then simply migrate the usages over? The sipDialinInfo is part of the signaling settings returned on GET /api/{apiVersion}/signaling/settings

spreed/lib/ResponseDefinitions.php

Line 323 in 4cb26cc

* sipDialinInfo: string,