Deprecate top_level_group? and test it in a Cop class

[corsonknowles]

In this PR, we add a deprecation message for top_level_group?, which has no callers in current Rubocop/RSpec.

top_level_group? was confirmed unused by @bquorning here:

• https://github.com/rubocop/rubocop-rspec/pull/1957#issuecomment-2370935206
• code change: https://github.com/rubocop/rubocop-rspec/pull/977/files#diff-538fd5bdf99dca6200224146c399b06a85bc9297598c1c4e71e0c9eaead8770bL15

We test this method using some stubbing in a Cop class that includes the mixin.

The specs can be removed along with the method in the next Major version update.

Split from:

• https://github.com/rubocop/rubocop-rspec/pull/1970

This PR serves the goal of completing line coverage for this repo and unblocks

• https://github.com/rubocop/rubocop-rspec/pull/1971

---

Before submitting the PR make sure the following are checked:

• [x] Feature branch is up-to-date with master (if not - rebase it).
• [x] Squashed related commits together.
• [x] Added tests.
• [x] Updated documentation.
• [x] Added an entry to the CHANGELOG.md if the new code introduces user-observable changes.
• [x] The build (bundle exec rake) passes (be sure to run this locally, since it may produce updated documentation that you will need to commit).

If you have created a new cop:

• [ ] Added the new cop to config/default.yml.
• [ ] The cop is configured as Enabled: pending in config/default.yml.
• [ ] The cop is configured as Enabled: true in .rubocop.yml.
• [ ] The cop documents examples of good and bad code.
• [ ] The tests assert both that bad code is reported and that good code is not reported.
• [ ] Set VersionAdded: "<<next>>" in default/config.yml.

If you have modified an existing cop's configuration options:

• [ ] Set VersionChanged: "<<next>>" in config/default.yml.