MDEV-25803 innodb.alter_candidate_key fix

There is a case when implicit primary key may be changed when removing
NOT NULL from the part of unique key. In that case we update
modified_primary_key which is then used to not skip key sorting.

According to is_candidate_key() there is no other cases when primary
kay may be changed implicitly.

sql/handler.h

@@ -426,6 +426,7 @@ enum enum_alter_inplace_result {
 #define HA_CREATE_TMP_ALTER     8U
 #define HA_LEX_CREATE_SEQUENCE  16U
 #define HA_VERSIONED_TABLE      32U
+#define HA_SKIP_KEY_SORT        64U
 
 #define HA_MAX_REC_LENGTH	65535