chore(docs): update dead link in Postgres docs (#915)

Update dead link in Postgres docs to external Postgres provider.

Author: jdpleiness

docs/admin/postgres.mdx

@@ -56,7 +56,7 @@ When running an [external PostgreSQL instance](/admin/external_services/postgres
   - [Heroku](https://devcenter.heroku.com/articles/upgrading-heroku-postgres-databases)
   - [EnterpriseDB](https://www.enterprisedb.com/docs/en/9.6/pg/upgrading.html)
   - [Citus](http://docs.citusdata.com/en/v8.1/admin_guide/upgrading_citus.html)
-  - [Aiven PostgreSQL](https://help.aiven.io/postgresql/operations/how-to-perform-a-postgresql-in-place-major-version-upgrade)
+  - [Aiven PostgreSQL](https://aiven.io/docs/platform/concepts/maintenance-window#related-pages)
   - [Your own PostgreSQL](https://www.postgresql.org/docs/16/pgupgrade.html)
 4. Turn Sourcegraph back on connecting to the now-upgraded database.