Pin NodeTestVersion to 21.2.0 again

because of nodejs/node#51081

Revert "[EngSys] unpin NodeTestVersion of 21.2.0 (Azure#27999)"

This reverts commit 884d750.

Author: jeremymeng

eng/pipelines/templates/stages/platform-matrix.json

@@ -20,7 +20,7 @@
     "NodeTestVersion": [
       "18.x",
       "20.x",
-      "21.x"
+      "21.2.0"
     ],
     "TestType": "node",
     "TestResultsFiles": "**/test-results.xml"