Force Travis to use PHP 8.0.9 for now #19100

Description:

We should at a later step consider to switch to PHP 8.1. But it seems currently the tests would fail there.

Review

• [ ] Functional review done
• [ ] Potential edge cases thought about (behavior of the code with strange input, with strange internal state or possible interactions with other Matomo subsystems)
• [ ] Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• [ ] Security review done
• [ ] Wording review done
• [ ] Code review done
• [ ] Tests were added if useful/possible
• [ ] Reviewed for breaking changes
• [ ] Developer changelog updated if needed
• [ ] Documentation added if needed
• [ ] Existing documentation updated if needed

@justinvelluppillai @bx80 @peterhashair I will merge this, as it will fix the failing tests for now. Switching to PHP 8.1 wasn't easily possible here, as tests were not passing. See https://app.travis-ci.com/github/matomo-org/matomo/builds/249369916
Guess we should plan in some time next milestone to switch tests to PHP 8.1 as it might take some more time to also fix the deprecation notices as well.

Be good if you can create an issue for that in the next milestone