Replace *_any_instance_of with *_next_instance_of in RSpec

Summary

Implement a RuboCop Cop to auto-correct automatically all the references that are able to be converted in RSpec as:

• expect_any_instance_of -> expect_next_instance_of
• allow_any_instance_of -> allow_next_instance_of

Why

The story starts with this comment about replacing allow_any_instance_of with expect_next_instance_of.

At that moment, I was surprised for the comment because:

• This is something specific to GitLab because we have the gotcha about avoiding using expect_any_instance_of or allow_any_instance_of in RSpec.
• We didn't have the allow_next_instance_of spec helper, so there is a discrepancy between the existing method (allow_any_instance_of) and its replacement (expect_next_instance_of).

I already created the allow_next_instance_of spec helper in !18139 (merged).

Improvements

I can see the following improvements:

• Developers don't need to point about style/gotchas that can be spotted automatically by tools (Rubocop).
• Follow our own guidelines (gotchas page) and dogfooding.
• Automatic replacement with Rubocop --auto-correct.