Suppress Lint/NonAtomicFileOperation offense

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

```console
% bundle exec rubocop
(snip)

Offenses:

tasks/changelog.rb:33:31: W: [Correctable] Lint/NonAtomicFileOperation:
Remove unnecessary existence checks Dir.exist?.
Dir.mkdir(ENTRIES_PATH) unless Dir.exist?(ENTRIES_PATH)
                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

115 files inspected, 1 offense detected, 1 offense autocorrectable
RuboCop failed!
```

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