kea: Upgrade to 2.6.4 release

kea: Upgrade to 2.6.4 release

The following bug fixes have been implemented since the previous stable
release, version 2.6.3:

1. **Syslog logging fix**: We corrected an issue in logging
configuration when parsing "syslog:<facility name>" [#3921, #3992].

2. **Fix for misconfigured global reservations**: Earlier Kea versions
could crash when handling misconfigured global reservations. The problem
is now fixed [#3910, #3911, #4001].

3. **Recent Sphinx support**: Support for recent versions of Sphinx, a
tool used to generate documentation, was added. This allows Kea
documentation and packages to be built on recent systems, while
maintaining compatibility with older versions. This fix was tested with
Sphinx 1.8.4 on Debian 10, 8.1.3 on Fedora 42, and 8.2.3 on ArchLinux
[#3800].

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>