MDEV-34209 InnoDB is disregarding read-only mode on slow shutdown

innobase_end(): Do not attempt to shrink the system tablespace if
innodb_read_only=ON or innodb_force_recovery>4. This fixes a regression
due to commit 2d6c2f22 (MDEV-32452).

This bug was caught when testing a fix of MDEV-34200, which adds
SET GLOBAL innodb_fast_shutdown=0 to the test innodb.undo_upgrade.

storage/innobase/handler/ha_innodb.cc

@@ -4262,7 +4262,7 @@ innobase_end(handlerton*, ha_panic_function)
 		}
 
 		/* Do system tablespace truncation during slow shutdown */
-		if (!srv_fast_shutdown
+		if (!srv_fast_shutdown && !high_level_read_only
 		    && srv_operation == SRV_OPERATION_NORMAL) {
 			fsp_system_tablespace_truncate();
 		}