While downgrades are not supported and misguided attempts at it could
cause serious corruption especially after
commit b07920b6
it might be useful if InnoDB would start up even after an upgrade to
MariaDB Server 11.0 or later had removed the change buffer.

innodb_change_buffering_update(): Disallow anything else than
innodb_change_buffering=none when the change buffer is corrupted.

ibuf_init_at_db_start(): Mention a possible downgrade in the corruption
error message. If innodb_change_buffering=none, ignore the error but do
not initialize ibuf.index.

ibuf_free_excess_pages(), ibuf_contract(), ibuf_merge_space(),
ibuf_update_max_tablespace_id(), ibuf_delete_for_discarded_space(),
ibuf_print(): Check for !ibuf.index.

ibuf_check_bitmap_on_import(): Remove some unnecessary code.
This function is only accessing change buffer bitmap pages in a
data file that is not attached to the rest of the database.
It is not accessing the change buffer tree itself, hence it does
not need any additional mutex protection.

This has been tested both by starting up MariaDB Server 10.8 on
a 11.0 data directory, and by running ./mtr --big-test while
ibuf_init_at_db_start() was tweaked to always fail.