Add changelog entry about major version upgrade

Co-authored-by: Kerry Carmichael <[email protected]>

Author: erthalion

release_notes/48-release-notes.adoc

@@ -42,7 +42,35 @@ Policy::
 
 This release adds improvements related to the following components and concepts:
 
-
+//ROX-20769
+[id="central-db-postgresql_15_{context}"]
+=== Central DB uses PostgreSQL 15
+
+The Central DB component now uses PostgreSQL 15, and {product-title-short} 4.8
+supports this version for external databases. A new installation with an
+internal database now uses this version by default. When upgrading an existing
+cluster to {product-title-short} 4.8, Central DB will perform an upgrade of the
+data it has collected.
+
+[IMPORTANT]
+====
+When preparing the upgrade to {product-title-short} 4.8, follow these
+suggestions:
+
+* Back up the database before upgrading to {product-title-short} 4.8
+
+* If you are not upgrading by using the Operator, check the disk space
+  available for the database by viewing the
+  `rox_central_postgres_available_size_bytes` metric. For the purposes of the
+  upgrade, the value should be double the amount of the already-consumed disk
+  space, as shown in the `rox_central_postgres_total_size_bytes` metric. If the
+  value is not correct, extend the database PVC.
+
+* Do not interrupt the upgrade procedure. If the upgrade is interrupted, a
+  manual intervention might be required to proceed.
+
+Depending on the amount of data, the upgrade can take extra time to finish.
+====
 
 [id="notable-technical-changes-rn-48_{context}"]
 == Notable technical changes