[3.13] gh-101100: Fix some Sphinx reference warnings in ``whatsnew/2.6.rst`` (GH-139236) (#139569)

gh-101100: Fix some Sphinx reference warnings in ``whatsnew/2.6.rst`` (GH-139236)
(cherry picked from commit 04a2f80a604ccb214902f35deebf0d075543737c)

Co-authored-by: rowanbudge <rowanbudge@gmail.com>
Co-authored-by: rowanvil <rowan@anvil.works>
Co-authored-by: Adam Turner <9087854+aa-turner@users.noreply.github.com>