Upgrade PHP Unit version

[Miraeld]

Is your feature request related to a problem? Please describe. It is not possible to run composer install while being on a PHP >= 8.0 as the PHPUnit rule we have in the composer.json requires a version <= PHP 7.4.

Describe the solution you'd like It would be nice to upgrade these version to keep up to date with php versions.

Describe alternatives you've considered Change these:

"phpunit/phpunit": "^7.5",
"wp-media/phpunit": "2.0"

to

"phpunit/phpunit": "^7.5 || ^8 || ^9",
"wp-media/phpunit": "^3"

[Miraeld]

Scope a solution

To implement this feature, we must change the version within the composer.json file. Also we shouldn't forget to trigger all tests to make sure everything works fine with the new version.

Development steps:

• [] Change composer.json
• [] Trigger all tests to make sure nothing breaks
• [] Make changes in case tests are broken.

Effort estimation:

S

Can be peer-coded:

No

Is a refactor needed in that part of the codebase?

No

[jeawhanlee]

Looks good to me...