Drop support for ruby 2.6

The net-imap dependency requires 2.7.3, to deal with kwargs.

Author: nevans

.github/workflows/test.yml

@@ -7,7 +7,7 @@ jobs:
     uses: ruby/actions/.github/workflows/ruby_versions.yml@master
     with:
       engine: cruby
-      min_version: 2.6
+      min_version: 2.7
 
   build:
     needs: ruby-versions

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