pageserver: require stripe size (#12257)

## Problem

In #12217, we began passing the stripe size in reattach responses, and
persisting it in the on-disk state. This is necessary to ensure the
storage controller and Pageserver have a consistent view of the intended
stripe size of unsharded tenants, which will be used for splits that do
not specify a stripe size. However, for backwards compatibility, these
stripe sizes were optional.

## Summary of changes

Make the stripe sizes required for reattach responses and on-disk
location configs. These will always be provided by the previous
(current) release.

libs/pageserver_api/src/upcall_api.rs

@@ -23,22 +23,12 @@ pub struct ReAttachRequest {
     pub register: Option<NodeRegisterRequest>,
 }
 
-fn default_mode() -> LocationConfigMode {
-    LocationConfigMode::AttachedSingle
-}
-
 #[derive(Serialize, Deserialize, Debug)]
 pub struct ReAttachResponseTenant {
     pub id: TenantShardId,
     /// Mandatory if LocationConfigMode is None or set to an Attached* mode
     pub r#gen: Option<u32>,
-
-    /// Default value only for backward compat: this field should be set
-    #[serde(default = "default_mode")]
     pub mode: LocationConfigMode,
-
-    // Default value only for backward compat: this field should be set
-    #[serde(default = "ShardStripeSize::default")]
     pub stripe_size: ShardStripeSize,
 }
 #[derive(Serialize, Deserialize)]