docs: add instructions for upgrading from 24.03 to 24.09 (#3096)

Backported-from: main (24.12)
Backported-to: 24.09
Backport-of: 3096

MIGRATION.md

@@ -6,6 +6,10 @@ Backend.AI Migration Guide
 * The migration should be done while the managers and agents are shut down.
 * This guide only describes additional steps to follow other than the code/package upgrades.
 
+# 24.03 to 24.09
+* PostgreSQL version upgraded from 15.1 to 16.3. Before upgrading, create a data dump, and restore the data after the upgrade is complete.
+* DB revision must be upgraded to `3596bc12ec09` **before** initiating Backend.AI manager Python package upgrade. Run `./backend.ai mgr schema apply-missing-revisions` after package upgrade.
+
 # 23.09 to 24.03
 * Python version upgraded from 3.11 to 3.12. Actual patch version may differ by every Backend.AI release, so please make sure to check `pants.toml` of each release.
 * DB revision must be downgraded to `85615e005fa3` **before** initiating Backend.AI manager Python package upgrade