Replace describe with RSpec.describe

Problem

Right now there are many places in the specs where we are using the monkey patched describe method and would be amazing to replace the first describe with RSpec.describe

Always using only the non-monkey patched `RSpec.describe`, instead of
inspecting the configuration, has the benefits of:

- not requiring RSpec to be loaded when the generators are run by Rails
- not introducing extra logic and state to handle the different states
- Resolve [#1048](https://github.com/rspec/rspec-rails/issues/1048) 

https://github.com/rspec/rspec-rails/commit/ca0d249858903949052e06884e8e7f9d596cdc79

Proposal

Replace all the first occurrence of describe in specs file with RSpec.describe