TASK: Drop code for PHP < 8

[kdambekalns]

This drops code that was only executed on PHP < 8.0, and thus will never ever be run again.
Checklist

• Code follows the PSR-2 coding style
• Tests have been created, run and adjusted as needed
• The PR is created against the lowest maintained branch
• Reviewer - PR Title is brief but complete and starts with FEATURE|TASK|BUGFIX
• Reviewer - The first section explains the change briefly for change-logs
• Reviewer - Breaking Changes are marked with !!! and have upgrade-instructions

[kdambekalns]

What's with the test failure? 🙄

[dlubitz]

See: #3409

Was merged right now, you just need to rebase.