Drop to Ruby 2.4

Author: hsbt

.github/workflows/test.yml

@@ -7,7 +7,7 @@ jobs:
     name: build (${{ matrix.ruby }} / ${{ matrix.os }})
     strategy:
       matrix:
-        ruby: [ 2.7, 2.6, 2.5, 2.4, head ]
+        ruby: [ 2.7, 2.6, 2.5, head ]
         os: [ ubuntu-latest, macos-latest ]
     runs-on: ${{ matrix.os }}
     steps:

net-imap.gemspec

@@ -13,7 +13,7 @@ Gem::Specification.new do |spec|
   spec.summary       = %q{Ruby client api for Internet Message Access Protocol}
   spec.description   = %q{Ruby client api for Internet Message Access Protocol}
   spec.homepage      = "https://github.com/ruby/net-imap"
-  spec.required_ruby_version = Gem::Requirement.new(">= 2.3.0")
+  spec.required_ruby_version = Gem::Requirement.new(">= 2.5.0")
 
   spec.metadata["homepage_uri"] = spec.homepage
   spec.metadata["source_code_uri"] = spec.homepage