cubicweb #2869426 remainings from the xhtml past [validation pending]

we dropped xhtml support in 3.17 but we keep providing documents using the XHTML strict doctype. And that is erroneous, as we dropped proper namespace definition as well as cubicweb DTD extensions.

We should switch to html(5) doctype.

prioritynormal
typebug
done in3.17.1
load1.000
load left0.000
closed by#aeb7d400ee92 [view] return HTML5 doctype. Closes #2869426
patch[view] return HTML5 doctype. Closes #2869426 [applied][view] return HTML5 doctype. Closes #2869426 [applied]