Add note on Azure flex server

doc/administration/troubleshooting/postgresql.md

@@ -107,6 +107,10 @@ This section is for links to information elsewhere in the GitLab documentation.
 
   To fix this, see [Backup and restore a non-packaged PostgreSQL database](https://docs.gitlab.com/omnibus/settings/database.html#backup-and-restore-a-non-packaged-postgresql-database).
 
+- Deploying PostgreSQL on Azure Database for PostgreSQL - Flexible Server may result in an error stating `extension "btree_gist" is not allow-listed for "azure_pg_admin" users in Azure Database for PostgreSQL`
+
+  To resolve the above error, [allow-list the extension](https://docs.microsoft.com/en-us/azure/postgresql/flexible-server/concepts-extensions#how-to-use-postgresql-extensions) prior to install.
+
 ## Support topics
 
 ### Database deadlocks