MDEV-31487: Recovery or backup failure after innodb_undo_log_truncate=ON

recv_sys_t::parse(): For undo tablespace truncation mini-transactions,
remember the start_lsn instead of the end LSN. This is what we expect
after commit 461402a5 (MDEV-30479).

storage/innobase/log/log0recv.cc

@@ -1951,9 +1951,9 @@ bool recv_sys_t::parse(lsn_t checkpoint_lsn, store_t *store, bool apply)
                         TRX_SYS_MAX_UNDO_SPACES, "compatibility");
           /* The entire undo tablespace will be reinitialized by
           innodb_undo_log_truncate=ON. Discard old log for all pages. */
-          trim({space_id, 0}, recovered_lsn);
+          trim({space_id, 0}, start_lsn);
           truncated_undo_spaces[space_id - srv_undo_space_id_start]=
-            { recovered_lsn, page_no };
+            { start_lsn, page_no };
           if (undo_space_trunc)
             undo_space_trunc(space_id);
 #endif