MDEV-25513: raise systemd LimitNOFILE limits to match server defaults

Quoting MDEV reporter Daniel Lewart:

Starting MariaDB with default configuration causes the following problems:

    "[Warning] Could not increase number of max_open_files to more than 16384 (request: 32186)"
    silently reduces table_open_cache_instances from 8 (default) to 4

Default Server System Variables:

    extra_max_connections = 1
    max_connections = 151
    table_open_cache = 2000
    table_open_cache_instances = 8
    thread_pool_size = 4

LimitNOFILE=16834 is in the following files:

    support-files/mariadb.service.in
    support-files/mariadb@.service.in

Looking at sql/mysqld.cc lines 3837-3917:
wanted_files= (extra_files + max_connections + extra_max_connections +
tc_size * 2 * tc_instances);
wanted_files+= threadpool_size;

Plugging in the default values:
wanted_files = (30 + 151 + 1 + 2000 * 2 * 8 + 4) = 32186

However, systemd configuration has LimitNOFILE = 16384, which is far smaller.

I suggest increasing LimitNOFILE to 32768.

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