Deprecated function errors in PHP 8.1

Problem/Motivation

Use phpstan tool was found deprecated function notice occurs in PHP 8.1
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Line /tmgmt/translators/file/tmgmt_file.recursive_iterator.inc
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42 Return type mixed of method RecursiveDOMIterator::current() is not covariant with tentative return type mixed of method Iterator::current().
52 Return type mixed of method RecursiveDOMIterator::getChildren() is not covariant with tentative return type RecursiveIterator|null of method RecursiveIterator::getChildren().
62 Return type mixed of method RecursiveDOMIterator::hasChildren() is not covariant with tentative return type bool of method
RecursiveIterator::hasChildren().
72 Return type mixed of method RecursiveDOMIterator::key() is not covariant with tentative return type mixed of method Iterator::key().
79 Return type mixed of method RecursiveDOMIterator::next() is not covariant with tentative return type void of method Iterator::next().
86 Return type mixed of method RecursiveDOMIterator::rewind() is not covariant with tentative return type void of method Iterator::rewind().
96 Return type mixed of method RecursiveDOMIterator::valid() is not covariant with tentative return type bool of method Iterator::valid().
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Proposed resolution

Update the function type of return value.