Small fixes after wxHtmlTagHandler::GetParser() addition.

Amend the changes of r73839:

- Don't use covariant return type for wxHtmlWinTagHandler::GetParser() as
  VC6, which we still support, doesn't like it. Besides, this seems
  unnecessary.

- Don't make GetParser() virtual at all as this doesn't appear to be necessary
  neither.

- Add "@since 2.9.5" to the documentation of the new method.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73849 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775