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

libs/pageserver_api/src/controller_api.rs

@@ -57,7 +57,7 @@ pub struct NodeRegisterRequest {
     pub listen_http_addr: String,
     pub listen_http_port: u16,
 
-    pub availability_zone_id: Option<String>,
+    pub availability_zone_id: String,
 }
 
 #[derive(Serialize, Deserialize)]