Bump minimum required PHP version to 7.2.

[anton-vlasenko]

What?

• Bumps the plugin's minimum PHP version from 7.0 to 7.2.
• Bumps PHPCS testVersion from 7.0- to 7.2-.
• Bumps the PHPUnit test Test with previous WP version from php: 7.0 to php: 7.2.

Fixes #60680.

Why?

WordPress Core parity.

WordPress 6.6.0's minimum supported PHP version is/will be 7.2.

How?

Modifies the plugin header's Requires PHP: from 7.0 to 7.2.

Testing Instructions

1. Change your local test environment to PHP 7.0 (7.1) and WordPress 6.6-alpha.
2. Install and activate the Gutenberg 18.1.x plugin.
3. Open the gutenberg.php file, and in the file's DocBlock change Requires PHP: back to 7.2.
   Expected: Stays activated, bulk actions disabled, and a notice appears:

This plugin does not work with your version of PHP. Learn more about updating PHP.

4. Deactivate the plugin.
   Expected: Bulk edit is disabled; Cannot Activate is displayed but not clickable; notice appears:

This plugin does not work with your version of PHP. Learn more about updating PHP.

[github-actions]

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Co-authored-by: anton-vlasenko <antonvlasenko@git.wordpress.org>
Co-authored-by: johnbillion <johnbillion@git.wordpress.org>
Co-authored-by: Soean <soean@git.wordpress.org>
Co-authored-by: talldan <talldanwp@git.wordpress.org>
Co-authored-by: ironprogrammer <ironprogrammer@git.wordpress.org>

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[anton-vlasenko]

Let's not merge this one now; I need some time to test it to ensure it works as expected.

This question needs to be discussed first: #60680 (comment)

[anton-vlasenko]

The PR has received two approvals.
I think this is sufficient consensus, considering the process is similar to #52344.
I will proceed with the version bump as soon as the CI jobs pass.

[anton-vlasenko]

Thank you for approving the PR, @johnbillion and @Soean!
The version has been bumped.

[colorful-tones]

Note: this is being included in the Gutenberg 18.2 release, because there was a conflict during release and it just seemed easier path forward.