docs: 3.20 -> 3.21 for legacy version name of v3

Change the legacy version name of DreamBBS-202X vx (v3.x) to 3.21.x
to indicate the actual year of its first release (2021),
which was planned to be 2020 but postponed.

VERSION

@@ -49,6 +49,6 @@ Copyright (C) 2004-2008                 Hung-Yi Chen (gaod)
 DreamLand Bulletin Board System         Version: 3.10 Rev. Beta 3
 Copyright (C) 2010-2017                 Pang-Wei Tsai (cache)
 
-DreamLand Bulletin Board System         Version: 3.10.95 - 3.20.0
+DreamLand Bulletin Board System         Version: 3.10.95 - 3.21.0
 Copyright (C) 2017-2021                 Wei-Cheng Yeh (IID)
                                         Sean Li-Shin Ho (r2)

include/timetype.h

@@ -1,5 +1,5 @@
 /*-------------------------------------------------------*/
-/* timetype.h   ( NCKU CCNS WindTop-DreamBBS 3.20 )      */
+/* timetype.h   ( NCKU CCNS WindTop-DreamBBS 3.21 )      */
 /*-------------------------------------------------------*/
 /* Author: Wei-Cheng Yeh (IID) <[email protected]>    */
 /* Target: Helper functions for fixed-size `time_t`      */

lib/mime.c

@@ -1,5 +1,5 @@
 /*-------------------------------------------------------*/
-/* lib/mime.c   ( NCKU CCNS WindTop-DreamBBS 3.20 )      */
+/* lib/mime.c   ( NCKU CCNS WindTop-DreamBBS 3.21 )      */
 /*-------------------------------------------------------*/
 /* Author: Wei-Cheng Yeh (IID) <[email protected]>    */
 /* Target: MIME code processing library for DreamBBS     */

lib/passwd.c

@@ -1,5 +1,5 @@
 /*-------------------------------------------------------*/
-/* lib/passwd.c ( NCKU CCNS WindTop-DreamBBS 3.20 )      */
+/* lib/passwd.c ( NCKU CCNS WindTop-DreamBBS 3.21 )      */
 /*-------------------------------------------------------*/
 /* Author: Wei-Cheng Yeh (IID) <[email protected]>    */
 /* Target: Password hashing library for DreamBBS         */