ci: Fetch Rails v8 tags

Author: toshimaru

.github/workflows/ci.yml

@@ -37,7 +37,7 @@ jobs:
         with:
           submodules: true
       - name: fetch Rails tags
-        run: cd rails && git fetch --depth=1 origin refs/tags/v7*:refs/tags/v7*
+        run: cd rails && git fetch --depth=1 origin refs/tags/v8*:refs/tags/v8*
       - uses: ruby/setup-ruby@v1
         with:
           ruby-version: 3.3