expat: Update to version 2.6.1

expat: Update to version 2.6.1

- Update from version 6.2.0 to 6.2.1
- Update of rootfile
- Changelog
    2.6.1 Thu February 29 2024
        Bug fixes:
            #817  Make tests independent of CPU speed, and thus more robust
       #828 #836  Expose billion laughs API with XML_DTD defined and
                    XML_GE undefined, regression from 2.6.0
        Other changes:
            #829  Hide test-only code behind new internal macro
            #833  Autotools: Reject expat_config.h.in defining SIZEOF_VOID_P
            #819  Address compiler warnings
       #832 #834  Version info bumped from 10:0:9 (libexpat*.so.1.9.0)
                    to 10:1:9 (libexpat*.so.1.9.1); see https://verbump.de/
                    for what these numbers do
        Infrastructure:
            #818  CI: Adapt to breaking changes in clang-format

Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>