Fix PHP 7 recommendation in migration section

[PypWalters]

Describe the bug

In the migration documentation it says:

Use PHP7
If PHP 7 is an option, use it.

This should either be upped to PHP 8, or removed entirely.

Steps to Reproduce

You can see the text here: https://github.com/10up/Engineering-Best-Practices/blob/gh-pages/_includes/markdown/Migrations.md?plain=1#L132

Screenshots, screen recording, code snippet

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Environment information

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WordPress information

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