fix: declaration of ArticleList::toHTMLWithCSS must be compatible with ArticleListUnique::toHTMLWithCSS

feat(ArticleListUnique): toHTML() can be passed to template engine and thus settings / variables can be passed on

Related: order-simple-checkout#12

(cherry picked from commit 21a31f8f)

Co-authored-by: Patrick Müller <p.mueller@pcsg.de>

Related: pcsg/buero#425

Related: pcsg/buero#344
Related: pcsg/buero#436

Related: pcsg/buero#344
Related: pcsg/buero#436

(cherry picked from commit 275e8d6c)

Co-authored-by: Michael Danielczok <michael@pcsg.de>

Code stability -> missing return types, qualifier imports, code cleanup, function converts to php8.1

related: #102

The ErpEntityInterface interface was changed in a previous commit.
We decided to declare this as a breaking change to create a version 3. with this commit pgpstan level 0 and 1 are also done.

BREAKING CHANGE: ErpEntityInterface extended