Replace deprecated set-output commands with environment files.

Update both date and api-ref outputs to use $GITHUB_OUTPUT instead of the deprecated ::set-output command.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: dblock

.github/workflows/update_api.yml

@@ -28,10 +28,10 @@ jobs:
         run: rm -rf vendor
       - name: Get current date
         id: date
-        run: echo "::set-output name=date::$(date +'%Y-%m-%d')"
+        run: echo "date=$(date +'%Y-%m-%d')" >> $GITHUB_OUTPUT
       - name: Get slack-api-ref ref
         id: api-ref
-        run: echo "::set-output name=api-ref::$(git rev-parse --short HEAD:lib/slack/web/api/slack-api-ref)"
+        run: echo "api-ref=$(git rev-parse --short HEAD:lib/slack/web/api/slack-api-ref)" >> $GITHUB_OUTPUT
       - name: GitHub App token
         if: ${{ github.repository == 'slack-ruby/slack-ruby-client' }}
         id: github_app_token