Hello Pflanzenoma,
The line is a border at the top of the .site-info panel that contains the site title and WordPress credit.
You can either move the border and hide the .site-info panel …
.site-info {
display: none;
}
.footer-navigation {
padding-top: var(--global--spacing-vertical);
border-top: 3px solid var(--global--color-border);
}
… or if you want to keep the .site-info you could use display: flex and flex-direction: column on the footer element to re-order the elements in the footer instead.
Oliver