Uninstall bundler before installing an old version

Author: etagwerker

.github/workflows/test.yml

@@ -22,6 +22,9 @@ jobs:
         with:
           ruby-version: ${{ matrix.ruby-version }}
           bundler-cache: true
+      - name: Uninstall existing Bundler
+        run: |
+          gem uninstall bundler -a -x || true
       - name: Install Bundler
         run: |
           if [[ "${{ matrix.ruby-version }}" == "2.6" || "${{ matrix.ruby-version }}" == "2.7" ]]; then