enchant2: upgrade to 2.7.3

Upstream NEWS:

  This release is made with Vala 0.56.17 (previous versions were made with
  0.56.0). This works around a bug in `valac` that generates code which causes
  an error in GCC >=14 and Clang >= 16.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>