[various] Update Swift plugins for non-nullable generics (#7742)
Updates Swift-based plugins to the current version of Pigeon:
- Removes workarounds for previous lack of support for non-nullable generics.
- Minor tweaks for other Pigeon changes in intervening versions.
These are done as a batch since the Swift plugins are unusual among our plugins in having strong typing changes on the native side, making them easier to do mechanically.
Part of https://github.com/flutter/flutter/issues/155891