CI: Downgrade to bundler < 2.4

More recent versions need Ruby >= 2.6.0.

Author: amotl

.github/workflows/tests.yml

@@ -40,7 +40,7 @@ jobs:
 
       - name: Bundle install
         run: |
-          gem install bundler
+          gem install bundler --version '< 2.4'
           bundle config path vendor/bundle
           bundle install