Add gc_blocking and restore latest_gc_cutoff in openapi spec (#10867)

## Problem

gc_blocking is missing in the tenant info, but cplane wants to use it.
Also, https://github.com/neondatabase/neon/pull/10707/ removed
latest_gc_cutoff from the spec, renaming it to applied_gc_cutoff.
Temporarily get it back until cplane migrates.

## Summary of changes

Add them.

ref https://neondb.slack.com/archives/C03438W3FLZ/p1739877734963979

libs/pageserver_api/src/models.rs

@@ -1080,8 +1080,7 @@ pub struct TenantInfo {
 
     /// Opaque explanation if gc is being blocked.
     ///
-    /// Only looked up for the individual tenant detail, not the listing. This is purely for
-    /// debugging, not included in openapi.
+    /// Only looked up for the individual tenant detail, not the listing.
     #[serde(skip_serializing_if = "Option::is_none")]
     pub gc_blocking: Option<String>,
 }

pageserver/src/http/openapi_spec.yml

@@ -882,6 +882,8 @@ components:
               properties:
                 reason:
                   type: string
+        gc_blocking:
+          type: string
 
     TenantCreateRequest:
       allOf:
@@ -1083,6 +1085,9 @@ components:
         min_readable_lsn:
           type: string
           format: hex
+        latest_gc_cutoff_lsn:
+          type: string
+          format: hex
         applied_gc_cutoff_lsn:
           type: string
           format: hex