Removed PHP 7.4 from builds in favor of PHP 8.1

To try and reduce the number of Travis builds, we are removing PHP 7.4 from the builds so that we're only using PHP 7.2 and 8.1 for builds.

[ ] 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

matomo-org / plugin-VisitorGenerator