deps: require Ruby 2.7+ instead of 2.4+, bc nokogiri needs it

We need nokogiri >= 1.14.3 to get the right behavior for namespace-looking tag names with ":" in them, and nokogiri 1.14.3 requires ruby >= 2.7. Ruby 2.7+ is all I want to work and test on for other reasons, anyway. Users who really want to deploy on older Rubies can edit the gemspec file.

But if I'm being honest, really my main reason for doing this is to make Rubocopy happy.

ronn-ng.gemspec

@@ -1,10 +1,8 @@
 Gem::Specification.new do |s|
   s.name = 'ronn-ng'
   s.version = '0.10.1.pre4'
-  # As of 2023-09, ronn-ng targets and is tested on Ruby 2.7 for deployment. It'll mostly
-  # work on lower versions, but in effect requires >= 2.7 because it needs nokogiri
-  # >= 1.14.3 for correct tag name handling, and that nokogiri requires Ruby 2.7.
-  s.required_ruby_version = '>= 2.4'
+  # We target Ruby 2.7+ for test & deploy, plus nokogiri 1.14.3+ need Ruby 2.7
+  s.required_ruby_version = '>= 2.7'
 
   s.summary     = 'Builds man pages from Markdown'
   s.description = 'Ronn-NG builds manuals in Unix man page and HTML format from Markdown. Ronn-NG is a modern, maintained fork of the original Ronn.'