fix: Add NodeTool to validation pipelines (#1555)

#### Details

This pull request adds NodeTool to make sure the validation pipeline use
Node 16.x. I noticed the canary validation pipelines started failing
yesterday with a Node version error; ubuntu-latest appears to be
shipping with Node v18.

##### Motivation

Fix validation pipelines.

##### Context

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#### Pull request checklist
<!-- If a checklist item is not applicable to this change, write "n/a"
in the checkbox -->
- [n/a] Addresses an existing issue: Fixes #0000
- [x] Added relevant unit test for your changes. (`yarn test`)
- [x] Verified code coverage for the changes made. Check coverage report
at: `<rootDir>/test-results/unit/coverage`
- [x] Ran precheckin (`yarn precheckin`)

pipelines/ado-extension-validation-template.yaml

@@ -16,6 +16,10 @@ variables:
     - group: 'ado-auth-example-secrets'
 
 steps:
+    - task: NodeTool@0
+      inputs:
+          versionSpec: '16.x'
+          displayName: Use Node 16.x
     # reused by all "url" cases
     - script: npx serve "$(System.DefaultWorkingDirectory)/dev/website-root" -l 5858 &
       displayName: 'Start /dev/website-root test server at http://localhost:5858'