Add installer support for avoiding MySQL utf8mb4_0900_ai_ci default collation

[Toutouwai]

If you install PW with MySQL 8.0+ then all tables default to utf8mb4_0900_ai_ci collation. This collation does not have wide support and causes issues if the database needs to be migrated to MariaDB. A utf8mb4_unicode_ci collation would be preferable.

Gemini says:

utf8mb4_0900_ai_ci is exclusively supported natively in MySQL 8.0+ and is not widely compatible across older legacy servers or MariaDB setups.
If you attempt to import a dump containing utf8mb4_0900_ai_ci into older MySQL engines (5.7 or lower) or standard MariaDB environments, you will trigger the common error: Error 1273 (HY000): Unknown collation: 'utf8mb4_0900_ai_ci'.

MariaDB & utf8mb4_0900_ai_ci Support
Standard MariaDB (10.x and early 11.x): Does not support utf8mb4_0900_ai_ci. MariaDB chose to skip Unicode 9.0 collations and eventually moved toward Unicode 14.0 (uca1400_...) instead.
Very Recent MariaDB (11.4.5+): MariaDB added select alias support for utf8mb4_0900_ai_ci specifically to simplify replication and imports from MySQL 8.0. However, relying on this requires both environments to run bleeding-edge versions, making it risky for general cross-platform portability.

Gemini recommended setting

$config->dbEngine = 'InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci';

or

$config->dbInitCommand = "SET NAMES 'utf8mb4' COLLATE 'utf8mb4_unicode_ci'";

but this is not obvious or easy to do via the PW installer.

Can we make it easier to achieve a widely supported utf8mb4_unicode_ci collation on install?