Fix PHP 8.1 deprecation warning #19103

Description:

Fixes deprecation notices as reported on https://forum.matomo.org/t/matomo-4-8-0-upgrade-warning-for-htmlspecialchars-using-php8-1/45056
refs #17686

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

This pull request has been mentioned on Matomo forums. There might be relevant details there:

https://forum.matomo.org/t/matomo-4-8-0-upgrade-warning-for-htmlspecialchars-using-php8-1/45056/4