Make major version 6.0 compatible with PHP 8.1

[llaville]

CI Unit tests report https://github.com/llaville/php-compatinfo/runs/4472349033?check_suite_focus=true

OK (221 tests, 615 assertions)

Remaining self deprecation notices (3)

  1x: Return type of Bartlett\CompatInfo\Application\Collection\SniffCollection::getIterator() should either be compatible with IteratorAggregate::getIterator(): Traversable, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice
    1x in ParameterTest::functionProvider from Bartlett\CompatInfo\Tests\Reference

  1x: Return type of Bartlett\CompatInfo\Application\Sniffs\KeywordBag::getIterator() should either be compatible with IteratorAggregate::getIterator(): Traversable, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice
    1x in ParameterTest::functionProvider from Bartlett\CompatInfo\Tests\Reference

  1x: Return type of Bartlett\CompatInfo\Application\Sniffs\KeywordBag::count() should either be compatible with Countable::count(): int, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice
    1x in ParameterTest::functionProvider from Bartlett\CompatInfo\Tests\Reference

[llaville]

Fixed with commit c049bfa