Fix outdated claim that continuous aggregate schemas can't change

TimescaleDB 2.28.0 (timescale/timescaledb#9825) adds ADD COLUMN on
continuous aggregates, so the note stating no schema changes are
possible is no longer accurate. Point it at the ALTER MATERIALIZED
VIEW reference.

Author: atovpeko

src/content/docs/build/performance-optimization/alter-update-table-schema.mdx

@@ -12,10 +12,11 @@ Schema changes on a {C.HYPERTABLE} use normal {C.PG} `ALTER TABLE` syntax, and {
 
 <Callout variant="note">
 
-While you can change the schema of an existing {C.HYPERTABLE}, you cannot change
-the schema of a {C.CAGG}. For {C.CAGG}s, the only
-permissible changes are renaming a view, setting a schema, changing the owner,
-and adjusting other parameters.
+Schema changes to a {C.CAGG} are limited. You can rename the view or its
+columns, set a schema, change the owner, and adjust parameters. Since
+{C.TIMESCALE_DB} 2.28.0, you can also add a new aggregate with
+[`ALTER MATERIALIZED VIEW ... ADD COLUMN`](/reference/timescaledb/continuous-aggregates/alter_materialized_view).
+Other schema changes are not supported.
 
 </Callout>