Lsn for get_timestamp_of_lsn should be string, not integer (#5840)

The `get_timestamp_of_lsn` pageserver endpoint has been added in #5497,
but the yml it added was wrong: the lsn is expected in hex format, not
in integer (decimal) format.

pageserver/src/http/openapi_spec.yml

@@ -352,7 +352,8 @@ paths:
           in: query
           required: true
           schema:
-            type: integer
+            type: string
+            format: hex
           description: A LSN to get the timestamp
       responses:
         "200":