WD-245: Add database best practices setup

[tormi]

Ticket WD-245

Changes

• removed deprecated MySQL database driver namespace,
• harmonized the naming of local database environment variables with Silta,
• Introduced a cleaner way to override Adminer's PHP-FPM settings via a configuration override file,
• added a best practice setting for Silta's MariaDB service.

Testing

Environment: https://feature-wd-245-mariadb-settings.drupal-project.dev.wdr.io

Checks

• [x] checkt there's no '\Drupal\Core\Database\Driver\mysql\Connection is deprecated in drupal:9.4.0 and is removed from drupal:11.0.0. warnings
• [x] check there's no [warning] Undefined array key "DB_NAME_DRUPAL" settings.php:11 or similar messages in deployment logs,
• [x] check if Adminer service still works,
• [x] check the Drupal status page for transaction isolation level, it should be READ COMMITTED.

[tormi]

check the Drupal status page for transaction isolation level, it should be READ COMMITTED.

This is still REPEATABLE-READ. @evilfurryone, could you please review 4d1b783?

[tormi]

@hkirsman, could you please review Adminer's 169b33b as clear_env = no was your brilliant discovery?

[tormi]

This is still REPEATABLE-READ

This is resolved after restarting the database pod.