[FIX] web: `background layout` should not apply margin-top on footer

1. `background layout` report template should keep the min-height property to
avoid breaking the pdf report template.
This property is needed to display the background image at the right position
(avoid image to be crop if not a lot of invoice lines). see https://github.com/odoo/odoo/commit/41c75ad0c37d567dd43e418fda927c7ae38a3add

2. margin-top can't be applied or it will also break the pdf layout.
`min-height` will ensure that the html rendered will be high enough to looks
like the real pdf invoice anyway.

opw-39876

addons/web/static/src/scss/layout_background.scss

@@ -19,7 +19,6 @@
     }
 }
 .o_background_footer {
-    margin-top: 200px;
     ul {
         border-top: 1px solid $o-brand-primary;
         border-bottom: 1px solid $o-brand-primary;
@@ -35,6 +34,7 @@
     background-size: cover;
     background-position: bottom center;
     background-repeat: no-repeat;
+    min-height: 900px;
     thead tr th, h2 {
         color: $o-brand-primary;
     }