textproc/libxml2: really update to 2.15.1

Changelog: https://gitlab.gnome.org/GNOME/libxml2/-/blob/2.15/NEWS

Notably, all documentation generation requires a circular dependency
on textproc/libxslt. While the API documentation, in HTML, lives
on the upstream site [0], the man pages are now pre-rendered manually.

Also includes an upstream commit addressing unsafe pointer usage
in XPath, leading to security bugs in consumers like libxslt
(CVE-2025-10911). [1]

Mark the -python child port DEPRECATED without expiration date since
upstream has not provided a firm one yet, particularly the release
schedule for the next major version 2.16.

[0] https://gnome.pages.gitlab.gnome.org/libxml2/html/index.html
[1] https://gitlab.gnome.org/GNOME/libxml2/-/commit/0e50b31902cdb1eb242eb361c123e9e033b2af87

Event: Snowstorm Special January 2026
With hat: desktop
PR: 291316