Make test suite work (#5770)

* Drop support for Rails < 7 and Ruby < 2.7
* Add tests to support to Rails 8+
* Fix test suite across all supported versions

test/support/webrat/matchers.rb

@@ -4,9 +4,13 @@ module Webrat
     class HaveSelector
       def query
         Nokogiri::CSS.parse(@expected.to_s).map do |ast|
-          ast.to_xpath("//", Nokogiri::CSS::XPathVisitor.new)
+          if ::Gem::Version.new(Nokogiri::VERSION) < ::Gem::Version.new('1.17.2')
+            ast.to_xpath('//', Nokogiri::CSS::XPathVisitor.new)
+          else
+            ast.to_xpath(Nokogiri::CSS::XPathVisitor.new)
+          end
         end.first
       end
     end
   end
-end
+end