Drop Ruby 2.6 support

The required IRB version only supports 2.7+

.github/workflows/protocol.yml

@@ -21,7 +21,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby-version: ['2.6', '2.7', '3.0', '3.1', 'head', 'debug']
+        ruby-version: ["2.7", "3.0", "3.1", "head", "debug"]
 
     steps:
     - uses: actions/checkout@v4

.github/workflows/ruby.yml

@@ -21,7 +21,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby-version: ['2.6', '2.7', '3.0', '3.1', '3.2', 'head', 'debug']
+        ruby-version: ["2.7", "3.0", "3.1", "3.2", "head", "debug"]
 
     steps:
     - uses: actions/checkout@v4

README.md

@@ -2,7 +2,7 @@
 
 # debug.rb
 
-This library provides debugging functionality to Ruby (MRI) 2.6 and later.
+This library provides debugging functionality to Ruby (MRI) 2.7 and later.
 
 This debug.rb is replacement of traditional lib/debug.rb standard library which is implemented by `set_trace_func`.
 New debug.rb has several advantages:

debug.gemspec

@@ -10,7 +10,7 @@ Gem::Specification.new do |spec|
   spec.description   = %q{Debugging functionality for Ruby. This is completely rewritten debug.rb which was contained by the ancient Ruby versions.}
   spec.homepage      = "https://github.com/ruby/debug"
   spec.licenses      = ["Ruby", "BSD-2-Clause"]
-  spec.required_ruby_version = Gem::Requirement.new(">= 2.6.0")
+  spec.required_ruby_version = Gem::Requirement.new(">= 2.7.0")
 
   spec.metadata["homepage_uri"] = spec.homepage
   spec.metadata["source_code_uri"] = spec.homepage

misc/README.md.erb

@@ -2,7 +2,7 @@
 
 # debug.rb
 
-This library provides debugging functionality to Ruby (MRI) 2.6 and later.
+This library provides debugging functionality to Ruby (MRI) 2.7 and later.
 
 This debug.rb is replacement of traditional lib/debug.rb standard library which is implemented by `set_trace_func`.
 New debug.rb has several advantages: