Fix: Increase @require-php to 7.2 since Gutenberg 18.2 requires it #5937

• Yes, I reviewed the contribution guidelines.
• Yes, more specifically, I reviewed the guidelines on how to write clear bug reports.

Describe the current, buggy behavior

Running the "Skip a theme using block patterns with Gutenberg active" test currently throws this warning:

Warning: The package could not be installed. "The PHP version on your server is 7.0.33-68+ubuntu20.04.1+deb.sury.org+2, however the uploaded plugin requires 7.2."
Warning: The 'gutenberg' plugin could not be found.

Describe what you expect as the correct outcome

The test should now break with a warning.