[skip changelog] Fix CI on Ruby 4.0 / Bump rake

ostruct is bundled on Ruby 4.0.
Somewhere in rake 13, it dropped it's dependency on ostruct.

Author: Earlopain

Gemfile

@@ -5,7 +5,7 @@ source "https://rubygems.org"
 # Specify your gem's dependencies in dead_end.gemspec
 gemspec
 
-gem "rake", "~> 12.0"
+gem "rake", "~> 13.0"
 gem "rspec", "~> 3.0"
 gem "stackprof"
 gem "standard"