Update minimum required version in PHP.

[peterwilsoncc]

What?

Updates the minimum required version in the admin notices and deactivation hooks.

Why?

To match the minimum version in the plugin header for consistency.

How?

• Introduces a constant for the minimum required WordPress version
• Updates the minimum version checks to use said constant
• Introduces a unit test to ensure the plugin header and new constant match

Let me know if you'd like me to define the value elsewhere or avoid using a constant.

Testing Instructions

1. Install a version of WordPress 6.4
2. Visit the dashboard
3. Ensure that the plugin deactivates and a notice is shown to the user.

Testing Instructions for Keyboard

Screenshots or screencast

[github-actions]

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Co-authored-by: peterwilsoncc <peterwilsoncc@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>

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[Mamaduka]

Thank you, @peterwilsoncc!