textproc/expat2: Update to 2.6.0

Fixes CVE-2023-52425 and CVE-2023-52426

Changelog:
https://github.com/libexpat/libexpat/blob/R_2_6_0/expat/Changes

References:
https://www.cve.org/CVERecord?id=CVE-2023-52425
https://www.cve.org/CVERecord?id=CVE-2023-52426

PR: 276946
Approved by: desktop (tcberner)
Exp-run by: antoine