fix ci: gem update with current version

.github/workflows/ci.yml

@@ -18,6 +18,10 @@ jobs:
     steps:
     - uses: actions/checkout@v3
 
+    - name: Get Ruby version
+      id: ruby-version
+      run: echo "::set-output name=VERSION::$(cat .ruby-version)"
+
     - name: Set up Ruby
       uses: ruby/setup-ruby@v1
 
@@ -65,7 +69,7 @@ jobs:
 
     - name: Set up RubyGems
       run: |
-        gem update --system --no-document --conservative
+        gem update --system ${{ steps.ruby-version.outputs.VERSION }} --no-document --conservative
         bundle config path vendor/bundle
         bundle install --jobs 4 --retry 3