[expat] Update version to 2.7.3 (#12340)

Change log: https://github.com/libexpat/libexpat/blob/R_2_7_3/expat/Changes

Author: mbauman

E/Expat/build_tarballs.jl

@@ -3,12 +3,12 @@
 using BinaryBuilder
 
 name = "Expat"
-version = v"2.7.1"
+version = v"2.7.3"
 
 # Collection of sources required to build Expat
 sources = [
     ArchiveSource("https://github.com/libexpat/libexpat/releases/download/R_$(version.major)_$(version.minor)_$(version.patch)/expat-$(version).tar.xz",
-                  "354552544b8f99012e5062f7d570ec77f14b412a3ff5c7d8d0dae62c0d217c30"),
+                  "71df8f40706a7bb0a80a5367079ea75d91da4f8c65c58ec59bcdfbf7decdab9f"),
 ]
 
 # Bash recipe for building across all platforms