Update to use enviorment variables

.github/workflows/mkdocs.yml

@@ -18,16 +18,16 @@ jobs:
         id: check_changes
         run: |
           if git diff --name-only ${{ github.event.before }} ${{ github.sha }} | grep -E 'mkdocs.yml|docs/'; then
-            echo "::set-output name=changes::true"
+            echo "CHANGES=true" >> $GITHUB_ENV
           else
-            echo "::set-output name=changes::false"
+            echo "CHANGES=false" >> $GITHUB_ENV
           fi
 
       - name: Install dependencies (if needed)
-        if: steps.check_changes.outputs.changes == 'true'
+        if: env.CHANGES == 'true'
         run: pip install mkdocs-material
 
       - name: Deploy (if needed)
-        if: steps.check_changes.outputs.changes == 'true'
+        if: env.CHANGES == 'true'
         run: mkdocs gh-deploy --force