docs: use GITHUB_OUTPUT in example workflow (#107)

* docs: use GITHUB_OUTPUT than set-output

see: https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/
Signed-off-by: Marco Lecheler <marco@task.media>

* docs: use actions/checkout@v3

Update example workflow to use latest version of checkout

Signed-off-by: Marco Lecheler <marco@task.media>

* fix: add quotes to GITHUB_OUTPUT

Co-authored-by: Carlos Tadeu Panato Junior <ctadeu@gmail.com>
Signed-off-by: Marco Lecheler <marco@task.media>

---------

Signed-off-by: Marco Lecheler <marco@task.media>
Co-authored-by: Carlos Tadeu Panato Junior <ctadeu@gmail.com>

README.md

@@ -37,7 +37,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
         with:
           fetch-depth: 0
 
@@ -59,7 +59,7 @@ jobs:
         run: |
           changed=$(ct list-changed --target-branch ${{ github.event.repository.default_branch }})
           if [[ -n "$changed" ]]; then
-            echo "::set-output name=changed::true"
+            echo "changed=true" >> "$GITHUB_OUTPUT"
           fi
 
       - name: Run chart-testing (lint)