Update DateTime and DateTimeImmutable signatures

[j4nr6n]

DateTime::format and DateTimeImmutable::format will always return a string starting in PHP 8.0.

https://psalm.dev/r/b5d797f092
https://www.php.net/manual/en/datetime.format.php#refsect1-datetime.format-changelog

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/b5d797f092

<?php

$theTime = new DateTimeImmutable();

$array = [
    $theTime->format('Y-m-d') => 'Y-m-d',
    $theTime->format('H:i:s') => 'H:i:s',
];

echo array_shift($array);

Psalm output (using commit 4cb1e2b):

ERROR: InvalidArrayOffset - 5:10 - Cannot create offset of type false|string, expecting array-key

[weirdan]

Thanks!