MDEV-29847: Wrong warning on rlimit capping of max_open_files: was: open_files_limit can no longer be set explicitly through LimitNOFILE

After upgrading from version 10.5.17 to version 10.6.10 the way open_files_limit is determined seems to have changed.
Before the open_files_limit could be set as follows:

1. joe /etc/sysctl.conf (set OS limit)

fs.file-max=500000

2. joe /etc/security/limits.conf (set mysql user limits)

mysql soft nofile 300000
mysql hard nofile 300000

3. joe /etc/systemd/system/mariadb.service.d/limits.conf (effectively set open_files_limit)

[Service]
LimitNOFILE=250000

Result:

MariaDB [(none)]> show variables like '%open_files_limit%';
------------------------+

------------------------+

------------------------+

After upgrading to version 10.6.10 all seemed well initially:

MariaDB [(none)]> show variables like '%open_files_limit%';
------------------------+

------------------------+

------------------------+

However, an error has now appeared in /var/log/messages on starting MariaDB:

[Warning] Could not increase number of max_open_files to more than 250000 (request: 262327)

Upon further investigation, the error disappears when changing the value of LimitNOFILE to a number larger than the requested 262327 mentioned in the warning:

3. joe /etc/systemd/system/mariadb.service.d/limits.conf

[Service]
LimitNOFILE=300000

However, the open_files_limit is now set to the aforementioned 262327 from the warning message in the log instead of the expected 250000:

MariaDB [(none)]> show variables like '%open_files_limit%';
------------------------+

------------------------+

------------------------+

This effective value for open_files_limit of 262327 seems to be based on the following settings in /etc/my.cnf:

table_open_cache = 16384
max_connections = 150

Roughly following this equation:
open_files_limit = table_open_cache * 16 + max_connections + (unclear remainder?)

Or in this case:
262327 = 16384 * 16 + 150 + (33?)

When adding 1000 to table_open_cache in /etc/my.cnf as follows:

table_open_cache = 17384
max_connections = 150

The result is:

MariaDB [(none)]> show variables like '%open_files_limit%';
------------------------+

------------------------+

------------------------+

Which is exactly 16000 more than the previous open_files_limit of 262327

When subsequently adding 10 to max_connections in /etc/my.cnf as follows:

table_open_cache = 17384
max_connections = 160

The result is:

MariaDB [(none)]> show variables like '%open_files_limit%';
------------------------+

------------------------+

------------------------+

Which is exactly 10 more than the previous open_files_limit of 278327

Conclusion:

Previously it was possible to explicitly specify open_files_limit through LimitNOFILE.

In the new situation:

1. If you specify a LimitNOFILE value smaller than 'table_open_cache * 16 + max_connections + (unclear remainder?)' the effective open_files_limit does equal the value for LimitNOFILE, but a warning is given in /var/log/messages that it could not be set to 'table_open_cache * 16 + max_connections + (unclear remainder?)'

2. If you specify a LimitNOFILE value larger than 'table_open_cache * 16 + max_connections + (unclear remainder?)' the effective open_files_limit does not equal the value for LimitNOFILE, but equals 'table_open_cache * 16 + max_connections + (unclear remainder?)'