doc: update tests and README to reflect libxml2 min req (#3304)

**What problem is this PR intended to solve?**

This should have been in 3bd6202

README.md

@@ -117,6 +117,10 @@ Requirements:
 - Ruby >= 3.0
 - JRuby >= 9.4.0.0
 
+If you are compiling the native extension against a system version of libxml2:
+
+- libxml2 >= 2.9.2 (recommended >= 2.12.0)
+
 
 ### Native Gems: Faster, more reliable installation
 

test/html4/test_document_fragment.rb

@@ -182,9 +182,6 @@ def test_fragment_with_comment
         end
 
         def test_element_children_counts
-          if Nokogiri.uses_libxml?("<= 2.9.1")
-            skip("#elements doesn't work in 2.9.1, see 1793a5a for history")
-          end
           doc = Nokogiri::HTML4::DocumentFragment.parse("   <div>  </div>\n   ")
           assert_equal(1, doc.element_children.count)
         end

test/xml/test_entity_reference.rb

@@ -377,15 +377,14 @@ def setup
         xml = File.read(xml_document)
         reader = Nokogiri::XML::Reader(xml, path, &:default_xml)
 
-        if Nokogiri.uses_libxml?(">= 2.9.0")
-          # Unknown entity is not fatal in libxml2 >= 2.9
-          assert_equal 8, reader.count
+        if Nokogiri.uses_libxml?
+          assert_equal(8, reader.count)
         else
           assert_raises(Nokogiri::XML::SyntaxError) do
             reader.count
           end
         end
-        assert_operator reader.errors.size, :>, 0
+        assert_operator(reader.errors.size, :>, 0)
       end
     end
   end