pageserver: remove legacy TimelineInfo::latest_gc_cutoff field (1/2) (#11149)

## Problem

This field was retained for backward compat only in
https://github.com/neondatabase/neon/pull/10707.

Once https://github.com/neondatabase/cloud/pull/25233 is released,
nothing external will be reading this field.

Internally, this was a mandatory field so storage controller is still
trying to decode it, so we must do this removal in two steps: this PR
makes the field optional, and after one release we can fully remove it.

Related: https://github.com/neondatabase/cloud/issues/24250

## Summary of changes

- Rename field to `_unused`
- Remove field from swagger
- Make field optional

libs/pageserver_api/src/models.rs

@@ -1225,9 +1225,10 @@ pub struct TimelineInfo {
     pub last_record_lsn: Lsn,
     pub prev_record_lsn: Option<Lsn>,
 
-    /// Legacy field for compat with control plane.  Synonym of `min_readable_lsn`.
-    /// TODO: remove once control plane no longer reads it.
-    pub latest_gc_cutoff_lsn: Lsn,
+    /// Legacy field, retained for one version to enable old storage controller to
+    /// decode (it was a mandatory field).
+    #[serde(default, rename = "latest_gc_cutoff_lsn")]
+    pub _unused: Lsn,
 
     /// The LSN up to which GC has advanced: older data may still exist but it is not available for clients.
     /// This LSN is not suitable for deciding where to create branches etc: use [`TimelineInfo::min_readable_lsn`] instead,