Stop trying to build the docs with more than one version

Signed-off-by: Pau Ruiz Safont <[email protected]>

.github/workflows/build.yml

@@ -12,7 +12,7 @@ jobs:
 
     strategy:
       matrix:
-        ruby-version: [2.6, 2.7, '3.0', 3.1]
+        ruby-version: [3.1]
 
     steps:
     - uses: actions/checkout@v2
@@ -29,10 +29,6 @@ jobs:
           gems-${{ runner.os }}-${{ matrix.ruby-version }}-
           gems-${{ runner.os }}-
 
-    # necessary to get ruby 2.3 to work nicely with bundler vendor/bundle cache
-    # can remove once ruby 2.3 is no longer supported
-    - run: gem update --system
-
     - run: bundle config set deployment 'true'
     - name: bundle install
       run: |