Update the default branch scheme to main

Author: shahmishal

utils/update_checkout/update-checkout-config.json

@@ -52,9 +52,9 @@
         "llvm-project": {
             "remote": { "id": "apple/llvm-project" } }
     },
-    "default-branch-scheme": "master",
+    "default-branch-scheme": "main",
     "branch-schemes": {
-       "master": {
+       "main": {
             "aliases": ["master", "swift/master", "main"],
             "repos": {
                 "llvm-project": "swift/master",