Lock bundler to 2.2.9 instead of latest

2.2.10 is causing the dependency resolution on Rails 6-0-stable to fail:

```
  Bundler could not find compatible versions for gem "railties":
    In Gemfile-rails-6-0:
      devise was resolved to 4.7.3, which depends on
        railties (>= 4.1.0)

      rails was resolved to 6.0.3.5, which depends on
        railties (= 6.0.3.5)

      responders (~> 3.0) was resolved to 3.0.1, which depends on
        railties (>= 5.0)
  Took  27.49 seconds
```

https://github.com/heartcombo/devise/runs/1905780158?check_suite_focus=true#step:5:23

The `railties` version 6.0.3.5 should work, given the other two are
using >= declarations, but it fails in 2.2.10.

Downgrading to 2.2.9 works.

.github/workflows/test.yml

@@ -100,7 +100,7 @@ jobs:
         with:
           ruby-version: ${{ matrix.ruby }}
           bundler-cache: true # runs bundle install and caches installed gems automatically
-          bundler: ${{ env.BUNDLER_VERSION || 'latest' }}
+          bundler: ${{ env.BUNDLER_VERSION || '2.2.9' }}
       - uses: supercharge/mongodb-github-action@1.3.0
         if: ${{ matrix.env == 'DEVISE_ORM=mongoid' }}
       - run: bundle exec rake