CI: Replace the deprecated `::set-output` command by writing to $GITHUB_OUTPUT in workflow definition

See "CI: Replace the deprecated `::set-output` command by writing to
$GITHUB_OUTPUT in matrix.py" for the reasoning behind this commit.
diff --git a/.github/workflows/windows.yml b/.github/workflows/windows.yml
index 3d03461..3030908 100644
--- a/.github/workflows/windows.yml
+++ b/.github/workflows/windows.yml
@@ -64,4 +64,4 @@
       id: show-version
       run: |
         ./haproxy -vv
-        echo "::set-output name=version::$(./haproxy -v |awk 'NR==1{print $3}')"
+        echo "version=$(./haproxy -v |awk 'NR==1{print $3}')" >> $GITHUB_OUTPUT