MDEV-28545 MyISAM reorganize partition corrupt older table format

The ALTER related code cannot do at the same time both:
- modify partitions
- change column data types

Explicit changing of a column data type together with a partition change is
prohibited by the parter, so this is not allowed and returns a syntax error:

  ALTER TABLE t MODIFY ts BIGINT, DROP PARTITION p1;

This fix additionally disables implicit data type upgrade
(e.g. from "MariaDB 5.3 TIME" to "MySQL 5.6 TIME", or the other way
around according to the current mysql56_temporal_format) in case of
an ALTER modifying partitions, e.g.:

  ALTER TABLE t DROP PARTITION p1;

In such commands now only the partition change happens, while
the data types stay unchanged.

One can additionally run:

  ALTER TABLE t FORCE;

either before or after the ALTER modifying partitions to
upgrade data types according to mysql56_temporal_format.

sql/sql_insert.cc

@@ -4346,6 +4346,16 @@ TABLE *select_create::create_table_from_items(THD *thd, List<Item> *items,
     alter_info->create_list.push_back(cr_field, thd->mem_root);
   }
 
+  /*
+    Item*::type_handler() always returns pointers to
+    type_handler_{time2|datetime2|timestamp2} no matter what
+    the current mysql56_temporal_format says.
+    Let's convert them according to mysql56_temporal_format.
+    QQ: This perhaps should eventually be fixed to have Item*::type_handler()
+    respect mysql56_temporal_format, and remove the upgrade from here.
+  */
+  Create_field::upgrade_data_types(alter_info->create_list);
+
   if (create_info->check_fields(thd, alter_info,
                                 create_table->table_name,
                                 create_table->db,