Support Ruby 2.5+ again

Author: hsbt

.github/workflows/test.yml

@@ -8,7 +8,7 @@ jobs:
     with:
       engine: cruby
       versions: '["truffleruby-head"]'
-      min_version: 2.7
+      min_version: 2.5
 
   build:
     needs: ruby-versions

zlib.gemspec

@@ -27,5 +27,5 @@ Gem::Specification.new do |spec|
   spec.executables   = []
   spec.require_paths = ["lib"]
   spec.extensions    = "ext/zlib/extconf.rb"
-  spec.required_ruby_version = ">= 2.3.0"
+  spec.required_ruby_version = ">= 2.5.0"
 end