chore(ci): upgrade github-pages-deploy-action

This upgrade was necessitated by the STDOUT `set-env` command's
deprecation.

Author: yvt

.github/workflows/doc.yml

@@ -51,7 +51,7 @@ jobs:
           mv target/doc output
 
       - name: Deploy
-        uses: JamesIves/github-pages-deploy-action@3.5.9
+        uses: JamesIves/github-pages-deploy-action@3.7.1
         with:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
           BRANCH: gh-pages