Move Riot-Defaults out of property lists

[aringenbach]

Your use case

Currently some Defaults are defined inside Riot-Defaults.plist, these would be better off somewhere else.

We need to be extra careful on migration because some of these defaults might be already set in RiotSettings.defaults with unexpected/historic values due to this migration: https://github.com/vector-im/element-ios/blob/10f250d7cd29a92cc257f5e07ae667580d27c946/Riot/Managers/Settings/RiotSettings.swift#L56

Have you considered any alternatives?

No

Additional context

This came up as an issue upon updating one of these defaults here: #6272