Revert "[rush] Bump pnpm in the rush init template to 10." (microsoft#5396)

* Revert "Bump pnpm in the rush init template to 10. (microsoft#5394)"

This reverts commit 71b7e0e.

* Rush change.

Author: iclanton

common/changes/@microsoft/rush/main_2025-10-03-14-59.json

@@ -0,0 +1,11 @@
+{
+  "changes": [
+    {
+      "comment": "",
+      "type": "none",
+      "packageName": "@microsoft/rush"
+    }
+  ],
+  "packageName": "@microsoft/rush",
+  "email": "iclanton@users.noreply.github.com"
+}

common/changes/@microsoft/rush/revert-5394-bump-init_2025-10-03-17-42.json

@@ -26,7 +26,7 @@
    * Specify one of: "pnpmVersion", "npmVersion", or "yarnVersion".  See the Rush documentation
    * for details about these alternatives.
    */
-  "pnpmVersion": "10.18.0",
+  "pnpmVersion": "9.15.9",
 
   /*[LINE "HYPOTHETICAL"]*/ "npmVersion": "6.14.15",
   /*[LINE "HYPOTHETICAL"]*/ "yarnVersion": "1.9.4",