MDEV-25951 MariaDB crash after ALTER TABLE convert to utf8mb4bb-10.4-MDEV-25951-instant-utf

Bug happens when partially indexed CHAR or VARCHAR field in converted from
utf8mb3 to utf8mb4.

Fixing by relaxing assertions. For some time dict_index_t and dict_table_t
are becoming not synchronized. Namely, dict_index_t has a new prefix_len which
is a multiple of a user-provided length and charset->mbmaxlen. But
the table still have and old mbmaxlen and assertion fails. This happens only
during utf8mb3 -> utf8mb4 conversions and the magic number 4 comes from
utf8mb_4_.

At the end of ALTER TABLE (innobase_rename_or_enlarge_columns_cache())
dict_index_t and dict_table_t became synchronized
again and will stay so at all times. For, example, they will be synchronized
on table load and newly added assertion proves that.

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