fix(admin): replace removed PHP ext/mysql INI example - #15719

Summary

Replace the outdated PHP ext/mysql / [mysql] INI example in the MySQL/MariaDB database setup page.

That extension was removed in PHP 7.0. Nextcloud requires PHP versions that only use mysqlnd + pdo_mysql. The page now shows modern drop-ins, links to the PHP manuals, and refers to pdo_mysql.default_socket instead of mysql.default_socket.

Closes #4758

Test plan

• Sphinx build of admin_manual succeeds for linux_database_configuration.rst
• No mysql.allow_* / removed ext/mysql keys remain in the PHP example
• Text mentions mysqlnd + pdo_mysql and points at php.net docs
• pdo_mysql.default_socket bullet replaces the old mysql.default_socket guidance

AI disclosure

This PR was drafted with the help of AI coding assistants (Cursor agents / LLM-based tools), including the description. I am responsible for it and happy to rework anything that doesn't fit.

All commits carry an Assisted-by: Cursor:grok-4.7 trailer.