chore: bump minimum Ruby to 3.3 and add Ruby 4.0 to CI matrix

Ruby 3.1 went EOL March 2025, Ruby 3.2 went EOL March 2026.
Update required_ruby_version to >= 3.3.0 and adjust the CI test
matrix to cover all currently supported Ruby versions: 3.3, 3.4, 4.0.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Signed-off-by: Zack Koppert <zkoppert@github.com>

Author: zkoppert

.github/workflows/ci.yml

@@ -14,9 +14,9 @@ jobs:
     strategy:
       matrix:
         ruby:
-          - "3.2"
           - "3.3"
           - "3.4"
+          - "4.0"
       fail-fast: false
 
     steps:

github-markup.gemspec

@@ -13,7 +13,7 @@ Gem::Specification.new do |s|
   s.homepage      = "https://github.com/github/markup"
   s.license       = "MIT"
 
-  s.required_ruby_version = '>= 3.1.0'
+  s.required_ruby_version = '>= 3.3.0'
 
   s.files         = `git ls-files`.split($\)
   s.executables   = s.files.grep(%r{^bin/}).map{ |f| File.basename(f) }