Use npm v8 instead of v9 on CI testing (#6464)

See the announcement about npm v9:
https://github.blog/changelog/2022-11-09-npm-cli-v9-is-now-generally-available/

See the failed job:
https://github.com/stylelint/stylelint/actions/runs/3433111899

.github/workflows/testing.yml

@@ -39,7 +39,8 @@ jobs:
           cache: npm
 
       - name: Install latest npm
-        run: npm install --global npm@latest
+        # TODO: npm@9 is the latest, but it doesn't work on Node.js v12 and v14 with windows.
+        run: npm install --global npm@8
 
       - name: Install dependencies
         run: npm ci