Rename files for Style/Strip cop

Follow up of ca0e4d3.

This PR fixes the following RuboCop offense.

```console
bundle exec rake internal_investigation

(snip)

Offenses:

spec/rubocop/cop/style/lstrip_rstrip_spec.rb:3:1: C: RSpec/FilePath:
Spec path should end with rubocop/cop/style/strip*_spec.rb.
RSpec.describe RuboCop::Cop::Style::Strip do
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

1198 files inspected, 1 offense detected
RuboCop failed!
Exited with code 1
```

https://circleci.com/gh/rubocop-hq/rubocop/43960

lib/rubocop.rb

@@ -458,7 +458,6 @@
 require_relative 'rubocop/cop/style/ip_addresses'
 require_relative 'rubocop/cop/style/lambda'
 require_relative 'rubocop/cop/style/lambda_call'
-require_relative 'rubocop/cop/style/lstrip_rstrip'
 require_relative 'rubocop/cop/style/line_end_concatenation'
 require_relative 'rubocop/cop/style/method_call_without_args_parentheses'
 require_relative 'rubocop/cop/style/method_call_with_args_parentheses'
@@ -529,6 +528,7 @@
 require_relative 'rubocop/cop/style/string_literals'
 require_relative 'rubocop/cop/style/string_literals_in_interpolation'
 require_relative 'rubocop/cop/style/string_methods'
+require_relative 'rubocop/cop/style/strip'
 require_relative 'rubocop/cop/style/struct_inheritance'
 require_relative 'rubocop/cop/style/symbol_array'
 require_relative 'rubocop/cop/style/symbol_literal'