[3.12] Doc: Fix default ``latex_elements['papersize']`` (GH-124525) (#124726)

Doc: Fix default ``latex_elements['papersize']`` (GH-124525)

https://www.sphinx-doc.org/en/master/latex.htmlGH-the-latex-elements-configuration-setting

It should be 'letterpaper' or 'a4paper' not 'letter' or 'a4'
(not to be confused with PAPER env variable).
(cherry picked from commit fae5058ec13aa3b4f1acc549fadfbbbc2628f1e9)

Co-authored-by: Jean-François B <2589111+jfbu@users.noreply.github.com>