feat: advertise content schema version 6

Add VERSION_6 carrying included_presets and set it as CONTENT_SCHEMA_VERSION
so new Kolibri knows an export carries the column. MIN_CONTENT_SCHEMA_VERSION
stays VERSION_5 since the column is additive and returned regardless.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: rtibblesbot

contentcuration/kolibri_content/constants/schema_versions.py

@@ -19,9 +19,12 @@
 
 VERSION_5 = "5"
 
+VERSION_6 = "6"
+
 # List of the content db schema versions, ordered from most recent to least recent.
 # When a new schema version is generated, it should be added here, at the top of the list.
 CONTENT_DB_SCHEMA_VERSIONS = [
+    VERSION_6,
     VERSION_5,
     VERSION_4,
     VERSION_3,
@@ -33,7 +36,7 @@
 ]
 
 # The latest compatible exported schema version for this version of Kolibri
-CONTENT_SCHEMA_VERSION = VERSION_5
+CONTENT_SCHEMA_VERSION = VERSION_6
 
 # The version name for the current content schema,
 # which may have schema modifications not present in the export schema