Update dependent RMagick version to 5.3.0

It has memory leak fix.
Ref. rmagick/rmagick#1406

gruff.gemspec

@@ -22,7 +22,7 @@ Gem::Specification.new do |s|
     s.platform = 'java'
     s.add_dependency 'rmagick4j'
   else
-    s.add_dependency 'rmagick', '>= 4.2'
+    s.add_dependency 'rmagick', '>= 5.3'
     s.add_development_dependency 'rubocop', '~> 1.50.2'
     s.add_development_dependency 'rubocop-minitest', '~> 0.30.0'
     s.add_development_dependency 'rubocop-performance', '~> 1.17.1'