Fix openssl_csr_export() signature

[mpesari]

The documentation for openssl_csr_export() was updated recently, although it does not seem to be live yet.

This fixes the 2nd parameter $output of openssl_csr_export() signature in Psalm.

Example of what this should fix (there should be no errors): https://psalm.dev/r/03755de763

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/03755de763

<?php

declare(strict_types=1);

$private_key = openssl_pkey_new(array(
    "private_key_bits" => 2048,
    "private_key_type" => OPENSSL_KEYTYPE_RSA,
));

$csr = openssl_csr_new([], $private_key);

openssl_csr_export($csr, $output);

print gettype($output); // "string"

$test = new class {
    public string $pem = '';
};

$test->pem = $output;

Psalm output (using commit 916fddb):

ERROR: InvalidPropertyAssignmentValue - 20:14 - $test->pem with declared type 'string' cannot be assigned type 'OpenSSLAsymmetricKey'

[orklah]

Thanks!