storcon: make pageserver AZ id mandatory (#8856)

## Problem
https://github.com/neondatabase/neon/pull/8852 introduced a new nullable
column for the `nodes` table: `availability_zone_id`

## Summary of changes
* Make neon local and the test suite always provide an az id
* Make the az id field in the ps registration request mandatory
* Migrate the column to non-nullable and adjust in memory state
accordingly
* Remove the code that was used to populate the az id for pre-existing
nodes

storage_controller/src/scheduler.rs

@@ -528,7 +528,7 @@ pub(crate) mod test_utils {
                         80 + i as u16,
                         format!("pghost-{i}"),
                         5432 + i as u16,
-                        None,
+                        "test-az".to_string(),
                     );
                     node.set_availability(NodeAvailability::Active(test_utilization::simple(0, 0)));
                     assert!(node.is_available());