wal-g: Use "start_time" field, not "time" which is S3 modified-at.

The `time` field is based on the file metadata in S3, which means that
touching the file contents in S3 can move backups around in the list.

Switch to using `start_time` as the sort key, which is based on the
contents of the JSON file stored as part of the backup, so is not
affected by changes in S3 metadata.

puppet/zulip/files/postgresql/wal-g-exporter

@@ -106,7 +106,7 @@ class WalGPrometheusServer(BaseHTTPRequestHandler):
             )
 
             if len(data) > 0:
-                data.sort(key=lambda e: e["time"], reverse=True)
+                data.sort(key=lambda e: e["start_time"], reverse=True)
                 latest = data[0]
                 labels = {
                     "host": latest["hostname"],