Support PHP 8+ development environments

[swissspidy]

Issue Overview

Due to the "phpunit/phpunit": "^7.5.20", version constraint (and perhaps also others) it's not currently possible to install dependencies on PHP 8.1

Using https://github.com/Yoast/wp-test-utils might be an easy way to solve this.

To Reproduce

Try to install dependencies and run tests on PHP 8 or 8.1

Expected behavior

I should be able to install dependencies and run tests on PHP 8+

Screenshots If applicable, add screenshots to help explain your problem.

Versions

• Browser:
• PHP: 8.1
• WP-CLI:
• GlotPress:
• WordPress:

Additional context

[ocean90]

I'm okay with bumping the PHP requirement to 8.1 based on https://www.php.net/supported-versions.php.