[8/21 Infra rollout] update sdk to preview6 (#56161)

[8/21 Infra rollout] update sdk to preview6 (#56161)

* update sdk to preview6

to check whether an arm64 bug is the root cause of NRE on macos.

* Remove invalid Castle.DynamicProxy.Internal.AbstractInvocation from ILLink descriptor files

The type is actually in the `Castle.DynamicProxy` namespace, not the `.Internal` one.
This causes an error with newer linker versions.

The whole `Castle.DynamicProxy` namespace is already preserved so we can remove the entry for AbstractInvocation.

Co-authored-by: Alexander Köplinger <alex.koeplinger@outlook.com>