Drop support for ruby 2.6

Author: nevans

.github/workflows/test.yml

@@ -7,7 +7,7 @@ jobs:
     name: build (${{ matrix.ruby }} / ${{ matrix.os }})
     strategy:
       matrix:
-        ruby: [ head, '3.2', '3.1', '3.0', '2.7', '2.6' ]
+        ruby: [ head, '3.2', '3.1', '3.0', '2.7']
         os: [ ubuntu-latest, macos-latest ]
     runs-on: ${{ matrix.os }}
     steps:

net-smtp.gemspec

@@ -17,7 +17,7 @@ Gem::Specification.new do |spec|
   spec.description   = %q{Simple Mail Transfer Protocol client library for Ruby.}
   spec.homepage      = "https://github.com/ruby/net-smtp"
   spec.licenses      = ["Ruby", "BSD-2-Clause"]
-  spec.required_ruby_version = ">= 2.6.0"
+  spec.required_ruby_version = ">= 2.7.3"
 
   spec.metadata["homepage_uri"] = spec.homepage
   spec.metadata["source_code_uri"] = spec.homepage