[Neos] Update guide for LTS version 7.3 and prevent error due to using PHP version 7.4

I will update the Neos guide for use with the new long term stability version 7.3 and include an update for the PHP version to use. Neos 7.3 supports PHP versions 7.4 and 8. Unfortunately there seems to be a bug with 7.4 where an internal Neos error will prevent users from uploading assets. Fortnunately using PHP 8 fixes this and is therefore the version to suggest using. 