Add MySQL 8.4 LTS testing.

[desrosj]

MySQL 8.4 was released on April 30, 2024 and is the latest LTS version of MySQL.

This adds MySQL testing to the GitHub Actions and eliminates the MySQL innovation releases since they are no longer supported or receiving security releases.

In order to properly test and support MySQL 8.4 in the local Docker environment while still allowing PHP 7.2 and 7.3 to be used, some changes were required to dynamically determine which authentication plugin to use and any additional configuration that is required.

This means that instead of blanket enforcing mysql_native_password, the environment will now always use the default unless that's unsupported (like caching_sha2_password for PHP 7.2/7.3).

Trac ticket: https://core.trac.wordpress.org/ticket/61218

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[github-actions]

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Props desrosj, jorbin.

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[github-actions]

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[desrosj]

@aristath I saw that you self assigned to perform a review. I think this is getting really close, so wanted to make sure you had a chance to take a look.

[aristath]

Thank you @desrosj!

Added a comment 👍