Add and apply Style/SingleLineMethods rubocop rule

Single line methods are hard to read, at least with this syntax. The
syntax used in Ruby 3.0 is accepted since Rubocop 1.7.0.

.rubocop.yml

@@ -517,6 +517,9 @@ Style/RedundantSelf:
 Style/SafeNavigation:
   Enabled: true
 
+Style/SingleLineMethods:
+  Enabled: true
+
 Style/SoleNestedConditional:
   Enabled: true
 

config/sitemap.rb

@@ -1,6 +1,9 @@
 # not use compression
 class SitemapGenerator::FileAdapter
-  def gzip(stream, data); stream.write(data); stream.close end
+  def gzip(stream, data)
+    stream.write(data)
+    stream.close
+  end
 end
 SitemapGenerator::Sitemap.namer = SitemapGenerator::SimpleNamer.new(:sitemap, extension: ".xml")