libxml2: upgrade 2.13.6 -> 2.13.8

libxml2: upgrade 2.13.6 -> 2.13.8

This includes CVE-fix for CVE-2025-32414 and CVE-2025-32415.

Changelog:
===========
https://gitlab.gnome.org/GNOME/libxml2/-/releases/v2.13.7
https://gitlab.gnome.org/GNOME/libxml2/-/releases/v2.13.8

Regressions

* tree: Fix xmlTextMerge with NULL args
* io: Fix compressed flag for uncompressed stdin
* parser: Fix parsing of DTD content

Security

* [CVE-2025-32415] schemas: Fix heap buffer overflow inxmlSchemaIDCFillNodeTables
* [CVE-2025-32414] python: Read at most len/4 characters. (Maks Verver)

Signed-off-by: Divya Chellam <divya.chellam@windriver.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>