gha: drop use of 'set-output'

Author: joshmoore

.github/workflows/build.yml

@@ -12,7 +12,7 @@ jobs:
         uses: actions/checkout@v3
       - name: List implementations
         id: set-matrix
-        run: echo "::set-output name=matrix::$(cd implementations && ls -1 | jq -ncR '[inputs]')"
+        run: echo "matrix=$(cd implementations && ls -1 | jq -ncR '[inputs]')" >> $GITHUB_OUTPUT
     outputs:
       matrix: ${{ steps.set-matrix.outputs.matrix }}