CascadingModelBinder: remove the IsFixed parameter

[SteveSandersonMS]

The new and as-yet unshipped CascadingModelBinder has an IsFixed parameter to control whether the name is allowed to vary. It turns out this is not required because there isn't any situation where the name would need to change dynamically.

By removing this flag, and making it behave as fixed in all cases:

• We always get the performance benefits of the underlying CascadingValue being flagged as fixed
• The implementation of CascadingModelBinder gets simplified a lot - much of the logic relating to checking what's changed and dynamically (re)creating the model binding context can be removed or turned into a simple one-time setup mechanism

We need to do this before .NET 8 ships to avoid it being a breaking change later.

[SteveSandersonMS]

Done in #49340