Merge pull request swiftlang#83163 from swiftlang/shahmishal-patch-2

Update the rebranch scheme to use stable/21.x

Author: bnbarham

utils/update_checkout/update-checkout-config.json

@@ -443,10 +443,10 @@
             }
         },
         "rebranch": {
-            "aliases": ["rebranch", "stable/20250601", "llvm.org/main"],
+            "aliases": ["rebranch", "stable/21.x", "llvm.org/main"],
             "repos": {
-                "llvm-project": "stable/20250601",
-                "swift-llvm-bindings": "llvm.org/main",
+                "llvm-project": "stable/21.x",
+                "swift-llvm-bindings": "stable/21.x",
                 "swift": "rebranch",
                 "cmark": "gfm",
                 "llbuild": "main",