Add and apply Lint/NonAtomicFileOperation rule

This rule was added in Rubocop 1.31.0; it follows the principles
mentioned in the Ruby Style Guide [1].

https://rubystyle.guide/#atomic-file-operations

.rubocop.yml

@@ -179,6 +179,9 @@ Lint/EmptyFile:
 Lint/LiteralAsCondition:
   Enabled: true
 
+Lint/NonAtomicFileOperation:
+  Enabled: true
+
 Lint/ParenthesesAsGroupedExpression:
   Enabled: true
 

spec/lib/tasks/sitemap_spec.rb

@@ -4,7 +4,7 @@ describe "rake sitemap:create", type: :system do
   let(:file) { Rails.root.join("public", "sitemap.xml") }
 
   before do
-    File.delete(file) if File.exist?(file)
+    FileUtils.rm_f(file)
     Rake::Task["sitemap:create"].reenable
   end