Add 6.0 migration guide to toctree

Author: ADmad

en/appendices/migration-guides.rst

@@ -11,4 +11,5 @@ each version and the migration path between 5.x minor releases.
     ./5-0-migration-guide
     ./5-1-migration-guide
     ./5-2-migration-guide
+    ./6-0-migration-guide
     ./phpunit10