[ruby/drb] 6d890ec5979ec72586dd5f66dd8d33f7a9aefd1e was introduced to support only Ruby 2.7+

https://github.com/ruby/drb/commit/bec410d184

lib/drb/drb.gemspec

@@ -13,7 +13,7 @@ Gem::Specification.new do |spec|
   spec.summary       = %q{Distributed object system for Ruby}
   spec.description   = %q{Distributed object system for Ruby}
   spec.homepage      = "https://github.com/ruby/drb"
-  spec.required_ruby_version = Gem::Requirement.new(">= 2.3.0")
+  spec.required_ruby_version = Gem::Requirement.new(">= 2.7.0")
   spec.licenses      = ["Ruby", "BSD-2-Clause"]
 
   spec.metadata["homepage_uri"] = spec.homepage