The way of ignoring alter-algorithm is inconsistent with other options and with itself

Note: Please do not rebase or force-push, just push a fix on top of the tree as a separate commit!

The alter_algorithm variable and option has become unused but preserved for backward compatibility.

When it is set at runtime, a warning is produced and the variable's value remains unchanged:

set alter_algorithm=COPY;

Warnings:

Warning	4200	The variable '@@alter_algorithm' is deleted and ignored. It it only exists for compatiblity with old installations

select @@alter_algorithm;

@@alter_algorithm

DEFAULT

However, if it is set upon startup, no warning is produced, and the value is actually set (even though ignored upon ALTER execution anyway):

# restart: --alter-algorithm=COPY

select @@alter_algorithm;

@@alter_algorithm

COPY

Even though we don't expect many users to have it in cnf file, there is no reason for such inconsistency. Besides, it is potentially dangerous, as it gives a user no hint that the option in the config no longer works.

In previous releases we already had other options which were made redundant but preserved for compatibility, but they work differently, see for example innodb-log-files-in-group.
If it's used on startup, it does produce a warning:

2024-03-20 16:17:31 0 [Warning] 'innodb-log-files-in-group' was removed. It does nothing now and exists only for compatibility with old my.cnf files.

and if it's used at runtime, it returns an error

MariaDB [test]> select @@innodb_log_files_in_group;

ERROR 1193 (HY000): Unknown system variable 'innodb_log_files_in_group'

So, the mechanism already exists and I think it would be sufficient to do the same for alter_algorithm. However, if there is a strong reason to make it produce a warning instead of an error at runtime, let it be so, but

1) produce a warning upon startup too, as it's the most important thing for an unused variable;
2) fix typos in the warning text

It it only exists for compatiblity with old installations

(remove extra "it" and correct "compatibility").

All above together in an MTR test:

--source include/have_innodb.inc

query_vertical select * from information_schema.system_variables where variable_name = 'ALTER_ALGORITHM';

set alter_algorithm=COPY;

select @@alter_algorithm;

--let $restart_parameters= --alter-algorithm=COPY

--source include/restart_mysqld.inc

select @@alter_algorithm;