github: replace set-output directive

The set-output directive in github action files is deprecated and will
be unsupported starting May 31st, so we remove it and replace it with a
redirection to the $GITHUB_OUTPUT file.

.github/workflows/acceptance-test.yml

@@ -24,7 +24,7 @@ jobs:
         # version, because "goenv" can react to it automatically.
         run: |
           echo "Building with Go $(cat .go-version)"
-          echo "::set-output name=go-version::$(cat .go-version)"
+          echo "name=go-version::$(cat .go-version)" >>"$GITHUB_OUTPUT"
   acceptance-test:
     runs-on: ubuntu-latest
     name: Acceptance Test