Exclude legacy RDoc < 4 fallback from coverage

The `else` branch is for RDoc < 4, which has been unsupported since
Ruby 2.4 (2016). Modern RDoc requires Options. The branch is unreachable
in any supported environment, so guard it with :nocov: markers rather
than contort the test suite to fake an unsupported RDoc version.

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

Author: zkoppert

lib/github/markup/rdoc.rb

@@ -13,7 +13,9 @@ def render(filename, content, options: {})
         if ::RDoc::VERSION.to_i >= 4
           h = ::RDoc::Markup::ToHtml.new(::RDoc::Options.new)
         else
+          # :nocov: RDoc < 4 has been unsupported since Ruby 2.4 (2016); modern RDoc requires Options.
           h = ::RDoc::Markup::ToHtml.new
+          # :nocov:
         end
         h.convert(content)
       end