PHP8 support

[simivar]

Is your feature request related to a problem? Please describe.

Currently, PHPWord is installable on PHP8 but the tests fail to run as it's installed with PHPUnit 4.8.36 which uses each() function removed from PHP in version 8.

Describe the solution you'd like

We have to allow PHPUnit of version ^8.0|^9.0 so we can run tests on PHP8 but... in version 8 return types were added so we have to as well remove support for PHP lower than 7.1.

• require PHPUnit >= 8.0
• require PHP >= 7.1

[liborm85]

PHP 8.0 is supported with following two hacks.

PHPUnit 8 is installed on PHP 8.0 now with this hack:
https://github.com/PHPOffice/PHPWord/blob/0.18.2/.github/workflows/ci.yml#L46-L47

And tests are transformed with this hack:
https://github.com/PHPOffice/PHPWord/blob/0.18.2/.github/workflows/ci.yml#L55-L63

After drop support PHP 5.x, 7.0 and 7.1 in library can be rewritten to a clean solution.