docs(developer): Document database deprecations due to upcoming doctrine update

Signed-off-by: Joas Schilling <coding@schilljs.com>

Author: nickvergessen

developer_manual/app_publishing_maintenance/app_upgrade_guide/upgrade_to_30.rst

@@ -124,5 +124,19 @@ Changed APIs
 Removed APIs
 ^^^^^^^^^^^^
 
+Deprecated APIs
+^^^^^^^^^^^^^^^
+
+- Calling ``OCP\DB\QueryBuilder\IExpressionBuilder::andX()`` without arguments is deprecated and will throw an exception in a future version as the underlying library is removing the functionality.
+- Calling ``OCP\DB\QueryBuilder\IExpressionBuilder::orX()`` without arguments is deprecated and will throw an exception in a future version as the underlying library is removing the functionality.
+- Calling ``OCP\DB\QueryBuilder\IQueryBuilder::delete()`` with ``$alias`` is deprecated and will throw an exception in a future version as the underlying library is removing the functionality.
+- Calling ``OCP\DB\QueryBuilder\IQueryBuilder::getQueryPart()`` is deprecated and will throw an exception in a future version as the underlying library is removing the functionality.
+- Calling ``OCP\DB\QueryBuilder\IQueryBuilder::getQueryParts()`` is deprecated and will throw an exception in a future version as the underlying library is removing the functionality.
+- Calling ``OCP\DB\QueryBuilder\IQueryBuilder::getState()`` is deprecated and will throw an exception in a future version as the underlying library is removing the functionality.
+- Calling ``OCP\DB\QueryBuilder\IQueryBuilder::resetQueryPart()`` is deprecated and will throw an exception in a future version as the underlying library is removing the functionality. Create a new query builder object instead.
+- Calling ``OCP\DB\QueryBuilder\IQueryBuilder::resetQueryParts()`` is deprecated and will throw an exception in a future version as the underlying library is removing the functionality. Create a new query builder object instead.
+- Calling ``OCP\DB\QueryBuilder\IQueryBuilder::update()`` with ``$alias`` is deprecated and will throw an exception in a future version as the underlying library is removing the functionality.
+- Calling ``OCP\IDBConnection::getDatabasePlatform()`` is deprecated and will throw an exception in a future version as the underlying library is renaming and removing platforms which breaks the backwards-compatibility. Use ``getDatabaseProvider()`` instead.
+
 Removed events
 ^^^^^^^^^^^^^^