Mark min_readable_lsn as required in the pageserver API spec (#11324)

Fully applies the changes of
https://github.com/neondatabase/cloud/pull/25233 to neon.git. The field
is always present in the Rust struct definition, so it can be marked as
required.

cc #10707

pageserver/src/http/openapi_spec.yml

@@ -1079,6 +1079,7 @@ components:
         - last_record_lsn
         - disk_consistent_lsn
         - state
+        - min_readable_lsn
       properties:
         timeline_id:
           type: string