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/schema.rs

@@ -25,7 +25,7 @@ diesel::table! {
         listen_http_port -> Int4,
         listen_pg_addr -> Varchar,
         listen_pg_port -> Int4,
-        availability_zone_id -> Nullable<Varchar>,
+        availability_zone_id -> Varchar,
     }
 }