CMakeLists.txt: update project version to 3.13.1

This was missed in the 3.13.0 release.

Bug: 441135035
Change-Id: I7549b3d318dd5522f10746c7994879e671891590

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index c7c513b..87e685d 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -23,7 +23,7 @@
 
 # VERSION is the release version of the library. It should be updated at the
 # same time as the LT_* variables.
-project(AOM LANGUAGES C CXX VERSION 3.12.0)
+project(AOM LANGUAGES C CXX VERSION 3.13.1)
 
 # GENERATED source property global visibility.
 if(POLICY CMP0118)