Bump plugin minimum supported PHP version to 7.0

[hellofromtonya]

Closes #52344

What?

• Bumps the plugin's minimum PHP version from 5.6 to 7.0.
• Bumps PHPCS testVersion from 5.6- to 7.0-.
• Bumps the PHPUnit test Test with previous WP version from php: 5.6 to php: 7.0.

Why?

WordPress Core parity.

WordPress 6.3.0's minimum supported PHP version is/will be 7.0.

How?

Modifies the plugin header's Requires PHP: from 5.6 to 7.0.

Testing Instructions

1. Change your local test environment to PHP 5.6 and WordPress 6.2.2.
2. Install and activate the Gutenberg 6.3.x plugin from the wp . org repo.
3. Open the gutenberg.php file, and in the file's DocBlock change Requires PHP: back to 7.0.
   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.

Screenshots

[github-actions]

Flaky tests detected in 657a8e5.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/5671454844
📝 Reported issues:

• [Flaky Test] Pick the additional variation in the inserted Columns block #47057 in specs/editor/plugins/block-variations.test.js
• [Flaky Test] should toggle "lightbox" in saved attributes #52350 in /test/e2e/specs/editor/blocks/image.spec.js
• [Flaky Test] zoom animation #52381 in /test/e2e/specs/editor/blocks/image.spec.js

[costdev]

Thanks for the PR @hellofromtonya!

Looks like that's all the references to 5.6 updated, other than the headers in test themes but I don't think there's any reason or benefit to bump those each time 😆

[hellofromtonya]

other than the headers in test themes but I don't think there's any reason or benefit to bump those each time

I debated whether to bump them or not. Like you said @costdev, I didn't see a benefit to bumping them.

[hellofromtonya]

Alrighty, seems to be enough consensus on the issue and approvals on this PR. Let's do the bump.

[hellofromtonya]

Thank you everyone! The version has been bumped 🎉