[FIX] portal: fix copyright color on the portal

Before this commit, for default colors, the text-muted class color was
set as !important from the portal and could not be overriden by the
website, leading to inconsistent display between the website and the
portal for the copyright.

As a fix for stable, we do not change the text-muted color from the
portal when using default colors.

task-2468472

closes odoo/odoo#71300

X-original-commit: d2e934631d4de02a7933434a1252dab994fed4b7
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>

addons/portal/static/src/scss/portal.scss

@@ -289,7 +289,7 @@ img, .media_iframe_video, .o_image {
 // To be applied to all portal pages if bg-color is white (default).
 @if ($o-portal-use-default-colors) {
     #wrapwrap.o_portal {
-        @include o-bg-color($o-portal-bg-color);
+        @include o-bg-color($o-portal-bg-color, $with-extras: false);
     }
 }