docs: update output usage in the readme (aws-actions#414)

based on the latest Github Action best practice: https://docs.github.com/en/actions/using-jobs/defining-outputs-for-jobs#example-defining-outputs-for-a-job

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

Author: amazingandyyy

README.md

@@ -83,7 +83,7 @@ The task definition file can be updated prior to deployment with the new contain
       run: |
         docker build -t $ECR_REGISTRY/$ECR_REPOSITORY:$IMAGE_TAG .
         docker push $ECR_REGISTRY/$ECR_REPOSITORY:$IMAGE_TAG
-        echo "::set-output name=image::$ECR_REGISTRY/$ECR_REPOSITORY:$IMAGE_TAG"
+        echo "image=$ECR_REGISTRY/$ECR_REPOSITORY:$IMAGE_TAG" >> $GITHUB_OUTPUT
 
     - name: Fill in the new image ID in the Amazon ECS task definition
       id: task-def