Readme: Update MySQL recommended version

[swissspidy]

Previously: #58158, #55791

MySQL 5.7 is reaching EOL in 10 days so we need to update the recommended version in both readme.html and the ​Requirements page on the website.

We should change it to 8.0, matching the ​https://make.wordpress.org/hosting/handbook/server-environment/#database page

Note that PHPUnit tests are currently failing because of that with:

readme.html's Recommended MySQL version is too old. Remember to update the WordPress.org Requirements page, too.
Failed asserting that '2023-10-21' is less than '2023-10-21'.

[SergeyBiryukov]

In 56982:

General: Bump the recommended MySQL version in readme.html.

MySQL 5.7 reaches EOL (“End of Life”) in October 2023. The recommended minimum is bumped to 8.0 for now.

References:

• ​MySQL Support Policies
• ​Hosting team handbook: Server Environment: Database

Follow-up to [31291], [33946], [35759], [52420], [52421], [54069].

Props swissspidy, SergeyBiryukov.
See #59701.

[SergeyBiryukov]

Keeping the ticket open for now to also update the ​Requirements page:

• ​Meta handbook: Updating content on WordPress.org
• ​WordPress.org Block Theme: Building Patterns from Page Content

Trac ticket: 59701

This was already fixed in https://core.trac.wordpress.org/changeset/56982

See https://core.trac.wordpress.org/ticket/59701#comment:5 where the recommended version in the WordPress readme was updated from 5.7 to 8.0, because version 5.7 reached EOL.

I don't know if this is the right place to make this change for the ​https://wordpress.org/about/requirements/ page though 🤷

Props @SergeyBiryukov

[swissspidy]

PR to update the Requirements page (I think): ​https://github.com/WordPress/wporg-main-2022/pull/396

[swissspidy]

Website is now updated and this is complete!