[ruby/set] Drop support for Ruby 2

https://github.com/ruby/set/commit/64dad673d8

lib/set.rb

@@ -216,7 +216,7 @@
 #   has been modified while an element in the set.
 #
 class Set
-  VERSION = "1.0.4"
+  VERSION = "1.1.0.dev"
 
   include Enumerable
 

lib/set/set.gemspec

@@ -15,7 +15,7 @@ Gem::Specification.new do |spec|
   spec.description   = %q{Provides a class to deal with collections of unordered, unique values}
   spec.homepage      = "https://github.com/ruby/set"
   spec.licenses      = ["Ruby", "BSD-2-Clause"]
-  spec.required_ruby_version = Gem::Requirement.new(">= 2.7.0")
+  spec.required_ruby_version = Gem::Requirement.new(">= 3.0.0")
 
   spec.metadata["homepage_uri"] = spec.homepage
   spec.metadata["source_code_uri"] = spec.homepage