Changed mac and ios node label

This changes the Mac and iOS job AMI that we're using to Big Sur (OSX 11.4 update). This includes the following based on https://github.com/aws-lumberyard/o3de/tree/development/scripts/build/build_node/Platform/Mac:

- XCode 12.4
- CMake 3.20.3

Note: As of 86136dd, these builds still fail with the same errors as the nightly build under Catalina. See SPEC-7273 and LYN-4071 for details

scripts/build/Platform/Mac/pipeline.json

@@ -1,6 +1,6 @@
 {
     "ENV": {
-        "NODE_LABEL": "mac-catalina-7ad2e45b",
+        "NODE_LABEL": "mac-bigsur-2fc3a22",
         "LY_3RDPARTY_PATH": "/Users/lybuilder/3rdParty",
         "TIMEOUT": 30,
         "WORKSPACE": "/Users/lybuilder/workspace",

scripts/build/Platform/iOS/pipeline.json

@@ -1,6 +1,6 @@
 {
     "ENV": {
-        "NODE_LABEL": "mac-catalina-7ad2e45b",
+        "NODE_LABEL": "mac-bigsur-2fc3a22",
         "LY_3RDPARTY_PATH": "/Users/lybuilder/3rdParty",
         "TIMEOUT": 30,
         "WORKSPACE": "/Users/lybuilder/workspace",