Description:
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An interval with ->f = 1.234 was previously understood as 1.234 second when using in DateTime::add() which sounds pretty consistent.

Now it's understood as 0.234 which feels pretty unsafe. Moreover, if we dump the interval, we see ->f value is still 1.234.

If ->f can now longer be >= 1, it would rather need to throw an exception rather than accepting it and be interpreted differently regarding where it's used.

Test script:
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$dateInterval = new DateInterval('PT0S');
$dateInterval->f = 1.234;

echo (new DateTimeImmutable('2000-01-01 00:00:00'))->add($dateInterval)->format('Y-m-d H:i:s.u');

Expected result:
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2000-01-01 00:00:01.234000

(What we got for PHP <= 8.0)

Actual result:
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2000-01-01 00:00:00.234000

(What we get now with master branch)