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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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

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Props desrosj, jorbin, aristath.

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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 👍

[johnbillion]

Looks good and all the previous comments have been addressed.

Reminder to not include the change to using the desrosj repo and branch for the reusable workflows.

[desrosj]

Merged in https://core.trac.wordpress.org/changeset/59279 and https://core.trac.wordpress.org/changeset/59280.