Suppress Lint/NonAtomicFileOperation offenses

This commit suppresss the following `Lint/NonAtomicFileOperation` offenses.

```console
Offenses:

spec/support/file_helper.rb:10:33: W: [Correctable]
Lint/NonAtomicFileOperation: Remove unnecessary existence checks File.exist?.
FileUtils.makedirs dir_path unless File.exist?(dir_path)
                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
tasks/changelog.rb:33:31: W: [Correctable] Lint/NonAtomicFileOperation:
Remove unnecessary existence checks Dir.exist?.
Dir.mkdir(ENTRIES_PATH) unless Dir.exist?(ENTRIES_PATH)
                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

257 files inspected, 2 offenses detected, 2 offenses autocorrectable
RuboCop failed!
```

Author: koic

spec/support/file_helper.rb

@@ -7,7 +7,7 @@ def create_file(file_path, content)
     file_path = File.expand_path(file_path)
 
     dir_path = File.dirname(file_path)
-    FileUtils.makedirs dir_path unless File.exist?(dir_path)
+    FileUtils.mkdir_p dir_path
 
     File.open(file_path, 'w') do |file|
       case content

tasks/changelog.rb

@@ -30,7 +30,7 @@ def initialize(type:, body: last_commit_title, ref_type: nil, ref_id: nil, user:
     end
 
     def write
-      Dir.mkdir(ENTRIES_PATH) unless Dir.exist?(ENTRIES_PATH)
+      FileUtils.mkdir_p(ENTRIES_PATH)
       File.write(path, content)
       path
     end