[#3564] Bump up version in configure.ac to 2.7.3

configure.ac

@@ -6,7 +6,7 @@ AC_PREREQ([2.69])
 # For released versions, this is in x.y.z format.
 # For GIT versions, this is x.y.z-git, where x.y.z denotes the next development
 # version that is worked on and that is to be released.
-AC_INIT(kea, 2.7.2-git, [email protected])
+AC_INIT(kea, 2.7.3-git, [email protected])
 AC_CONFIG_SRCDIR(README)
 
 # serial-tests is not available in automake version before 1.13, so