Drop support for Ruby 2.6

Author: taketo1113

.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
 
   test:
     needs: ruby-versions

net-http.gemspec

@@ -21,7 +21,7 @@ Gem::Specification.new do |spec|
   spec.summary       = %q{HTTP client api for Ruby.}
   spec.description   = %q{HTTP client api for Ruby.}
   spec.homepage      = "https://github.com/ruby/net-http"
-  spec.required_ruby_version = Gem::Requirement.new(">= 2.6.0")
+  spec.required_ruby_version = Gem::Requirement.new(">= 2.7.0")
   spec.licenses      = ["Ruby", "BSD-2-Clause"]
 
   spec.metadata["changelog_uri"] = spec.homepage + "/releases"