Used SimplePie (1.2.1-dev) is not fully php8 compatible

ISPconfig 3.3.1 still uses SimplePie 1.2.1-dev (~2011), which is not php8 compatible.
Strangely (to me at least) your own feed is shown without any problems (afaik), but the feed https://www.heise.de/security/feed.xml throws the following error:

PHP Fatal error:  Uncaught Error: Non-static method SimplePie_Misc::atom_10_content_construct_type() cannot be called statically in /usr/local/ispconfig/interface/lib/classes/simplepie.inc.php:3349

while the feed is not shown.

Declaring the functions:

function atom_10_content_construct_type($attribs)
function atom_03_construct_type($attribs)

as

public static function atom_10_content_construct_type($attribs)
public static function atom_03_construct_type($attribs)

fixes that error and shows the feed.