Add a placeholder for next version in CHANGELOG

This allows us to start describing changes in the next libaom release.

Note: The second word in this line must not start with "v", otherwise
build/cmake/version.pl will parse the second word as the current
version.

Change-Id: I3a108e666f6c42e120e5e300c454d3ffe3ea4dcc

CHANGELOG

diff --git a/CHANGELOG b/CHANGELOG
index 76d8706..6d0cb69 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,3 +1,5 @@
+2025-mm-dd next version
+
 2025-04-11 v3.12.1
   This release includes several bug fixes. This release is ABI
   compatible with the last release. See