ci: remove deprecated set-output

Author: morpheu

.github/workflows/ci.yaml

@@ -66,7 +66,7 @@ jobs:
         elif [[ ${ref} =~ ^${{ matrix.platform }}-([0-9.]+)$ ]]; then
           tags=${img}:${BASH_REMATCH[1]}-${{ matrix.arch }},${img}:latest-${{ matrix.arch }}
         fi
-        echo "::set-output name=tag::${tags}"
+        echo "tag=${tags}" >> $GITHUB_OUTPUT
       if: github.event_name != 'pull_request'
       id: tags
       shell: bash