Replace our nullability decoding with the new BCL API

[roji]

One possible design question...

• NRT nullability is computed in a convention, but not value type nullability; that's just hard-coded in Property.DefaultIsNullable.
  • We want NRT computation to be in a convention, since there's a NullabilityInfoContext which caches stuff for perf.
  • We could have a single convention setting IsRequired for both NRTs and value types (NullabilityInfoContext detects for those too), but this could possibly be heavier perf-wise. Also not sure it's worth it, /cc @AndriySvyryd.

Also see #26966 which was discovered as part of this.

Closes #24744

[AndriySvyryd]

We could have a single convention setting IsRequired for both NRTs and value types (NullabilityInfoContext detects for those too), but this could possibly be heavier perf-wise. Also not sure it's worth it, /cc @AndriySvyryd.

I'll consider this as part of #214