Add a stub update-version.yml file for testing.

GitHub Action workflows can't be tested until they exist on main in some form. This PR adds an empty copy of update-version.yml in order to allow me to test it. (We can delete this file if we don't move forward with merging the actual workflow.)

Author: grynspan

.github/workflows/update-version.yml

@@ -0,0 +1,12 @@
+# This source file is part of the Swift.org open source project
+#
+# Copyright (c) 2025 Apple Inc. and the Swift project authors
+# Licensed under Apache License v2.0 with Runtime Library Exception
+#
+# See https://swift.org/LICENSE.txt for license information
+# See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+
+name: Update Swift Testing version
+
+on:
+  pull_request: