Drop Ruby 2.5 / 2.6 support

In the past CI was adjusted to Require Ruby 2.7 or newer, but we didn't
drop the old versions from the gemspec. While it claims to support Ruby
2.5 & 2.6, it doesn't work in reality. This changes fixes this.

Ruby 2.5 was used in puppet AIO 6 / and jruby 9.3 (compatible with Ruby
2.6) in puppetserver 7. Both are EoL since ages.

Author: bastelfreak

.rubocop.yml

@@ -2,7 +2,7 @@
 inherit_from: .rubocop_todo.yml
 
 AllCops:
-  TargetRubyVersion: 2.5
+  TargetRubyVersion: 2.7
   Exclude:
     - acceptance/**/*
     - vendor/**/*

openfact.gemspec

@@ -24,7 +24,7 @@ Gem::Specification.new do |spec|
   base = "#{__dir__}#{File::SEPARATOR}"
   spec.files = dirs.map { |path| path.sub(base, '') }
 
-  spec.required_ruby_version = '>= 2.5', '< 5.0'
+  spec.required_ruby_version = '>= 2.7', '< 5.0'
   spec.bindir = 'bin'
   spec.executables   = spec.files.grep(%r{^bin/}) { |f| File.basename(f) }
   spec.require_paths = ['lib']