[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>

Author: mangod9

global.json

@@ -1,11 +1,11 @@
 {
   "sdk": {
-    "version": "6.0.100-preview.5.21302.13",
+    "version": "6.0.100-preview.6.21355.2",
     "allowPrerelease": true,
     "rollForward": "major"
   },
   "tools": {
-    "dotnet": "6.0.100-preview.5.21302.13"
+    "dotnet": "6.0.100-preview.6.21355.2"
   },
   "native-tools": {
     "cmake": "3.16.4",