Mark unsafe for Lint/RaiseException cop

[koic]

Follow up #8169 (comment).

Lint/RaiseException cop is unsafe due to incompatibility between bad and good cases. The following is an example.

bad case

% cat /tmp/example_for_exception.rb
begin
  raise Exception
rescue
  puts 'Handle exception'
end

% ruby example_for_exception.rb
example_for_exception.rb:2:in `<main>': Exception (Exception)

good case

% cat example_for_starndard_error.rb
begin
  raise StandardError
rescue
  puts 'Handle exception'
end

% ruby /tmp/example_for_starndard_error.rb
Handle exception

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[bbatsov]

Thanks!