expat: Update to version 2.6.4

expat: Update to version 2.6.4

- Update from version 2.6.3 to 2.6.4
- Update of rootfile
- Changelog
    2.6.4
        Security fixes:
            #915  CVE-2024-50602 -- Fix crash within function XML_ResumeParser
                    from a NULL pointer dereference by disallowing function
                    XML_StopParser to (stop or) suspend an unstarted parser.
                    A new error code XML_ERROR_NOT_STARTED was introduced to
                    properly communicate this situation.  // CWE-476 CWE-754
        Other changes:
            #903  CMake: Add alias target "expat::expat"
            #905  docs: Document use via CMake >=3.18 with FetchContent
                    and SOURCE_SUBDIR and its consequences
            #902  tests: Reduce use of global parser instance
            #904  tests: Resolve duplicate handler
       #317 #918  tests: Improve tests on doctype closing (ex CVE-2019-15903)
            #914  Fix signedness of format strings
       #919 #920  Version info bumped from 10:3:9 (libexpat*.so.1.9.3)
                    to 11:0:10 (libexpat*.so.1.10.0); see https://verbump.de/
                    for what these numbers do
        Infrastructure:
            #907  CI: Upgrade Clang from 18 to 19
            #913  CI: Drop macos-12 and add macos-15
            #910  CI: Adapt to breaking changes in GitHub Actions
            #898  Add missing entries to .gitignore

Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Arne Fitzenreiter <arne_f@ipfire.org>