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

pageserver/src/http/openapi_spec.yml

@@ -1079,7 +1079,6 @@ components:
         - last_record_lsn
         - disk_consistent_lsn
         - state
-        - latest_gc_cutoff_lsn
       properties:
         timeline_id:
           type: string
@@ -1123,9 +1122,6 @@ components:
         min_readable_lsn:
           type: string
           format: hex
-        latest_gc_cutoff_lsn:
-          type: string
-          format: hex
         applied_gc_cutoff_lsn:
           type: string
           format: hex

pageserver/src/http/routes.rs

@@ -460,10 +460,7 @@ async fn build_timeline_info_common(
         initdb_lsn,
         last_record_lsn,
         prev_record_lsn: Some(timeline.get_prev_record_lsn()),
-        // Externally, expose the lowest LSN that can be used to create a branch as the "GC cutoff", although internally
-        // we distinguish between the "planned" GC cutoff (PITR point) and the "latest" GC cutoff (where we
-        // actually trimmed data to), which can pass each other when PITR is changed.
-        latest_gc_cutoff_lsn: min_readable_lsn,
+        _unused: Default::default(), // Unused, for legacy decode only
         min_readable_lsn,
         applied_gc_cutoff_lsn: *timeline.get_applied_gc_cutoff_lsn(),
         current_logical_size: current_logical_size.size_dont_care_about_accuracy(),