Explictly require hangouts-chat in Gemfile

After the model `HangoutsChatService` was renamed
`Integrations::HangoutsChat`, and therefore the file renamed as
`hangsouts_chat.rb`,
`config/initializers/hangouts_chat_http_override.rb` began to define new
classes, rather than to monkeypatch the gem classes.

Requiring the gem this way allows us to continue to monkeypatch the gem.

https://gitlab.com/gitlab-org/gitlab/-/merge_requests/61957#note_586260689
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/61957
https://gitlab.com/gitlab-org/gitlab/-/issues/201855