Select PHP version for phpstan

[rdwebdesign]

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What does this PR aim to accomplish?:

This PR fixes the errors in phpstan tests, introduced by the new Virtual Environments forcing PHP8.1 (after 2021-12-08).

How does this PR accomplish the above?:

Creating a new step for phpstan to select the correct PHP version for the tests.

What documentation changes (if any) are needed to support this PR?:

none

[DL6ER]

Pinning the version to PHP 8.0 seems to be a good solution for now.

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/pi-hole-ftl-v5-12-web-v5-9-and-core-v5-7-released/51795/1