os/mac/pkgconfig/15: update expat version for 15.2

[cho-m]

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Saw failure in another PR probably from actions/runner-images@45226c1

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❯ grep VERSION /Library/Developer/CommandLineTools/SDKs/MacOSX15.2.sdk/usr/include/expat_config.h
#define PACKAGE_VERSION "2.6.3"
#define VERSION "2.6.3"