Install bundler versions depending on Ruby version (#41962)

Summary:
Since yesterday evening (why it is always friday evening???) CircleCI or Gem decided to update the default bundler version that is installed with `gem bundle install`.
Therefore, CI for iOS stopped working.

This change installs bundler's versions so that they are compatible with the Ruby version.

[Internal] - Fix CI for iOS installing versions of bundler that are compatible with Ruby

Pull Request resolved: #41962

Test Plan: CircleCI is green

Reviewed By: GijsWeterings

Differential Revision: D52230544

Pulled By: cipolleschi

fbshipit-source-id: 2f96e16ecb94159953056e8de757ea4d249f80f0

.circleci/config.yml

@@ -202,7 +202,11 @@ commands:
 
             # Set ruby dependencies
             rbenv global << parameters.ruby_version >>
-            gem install bundler
+            if [[ << parameters.ruby_version >> == "2.6.10" ]]; then
+              gem install bundler -v 2.4.22
+            else
+              gem install bundler
+            fi
             bundle check || bundle install --path vendor/bundle --clean
       - save_cache:
           key: *rbenv_cache_key