Pin the Hugo version to address incompatibility with Docsy template

The docsy template has not yet been updated to work with hugo v0.125+:  google/docsy#1973. This pins the GHA workflow to an earlier version to address the incompatibility.

.github/workflows/gh-pages.yml

@@ -19,7 +19,7 @@ jobs:
       - name: Setup Hugo
         uses: peaceiris/actions-hugo@v2
         with:
-          hugo-version: 'latest'
+          hugo-version: 0.121.2
           extended: true
       - name: Install Dependencies
         run: npm install autoprefixer postcss postcss-cli