[FIX] web: BS4, main component grayscale

BS4 blue-ish grayscale is ok for text, borders and shadows but it's too saturate for main components with solid background (eg. control panel and body bg).
This commit will use a desaturate version of the same colors.

addons/web/static/src/scss/kanban_view.scss

@@ -368,7 +368,7 @@
     &.o_kanban_grouped {
         min-height: 100%;
         padding: 0;
-        background-color: theme-color('light');
+        background-color: desaturate($gray-100, 100%);
 
         .o_kanban_record, .o_kanban_quick_create {
             width: 100%;
@@ -450,6 +450,7 @@
     .o_kanban_group.o_column_folded {
         @include o-kanban-slim-col;
         background-color: #f0f0f0;
+        background-color: desaturate($gray-200, 100%);
 
         & + .o_kanban_group.o_column_folded {
             margin-left: 1px;

addons/web/static/src/scss/secondary_variables.scss

-$o-webclient-background-color: $o-gray-100;
-$o-control-panel-background-color: $o-gray-100;
+$o-webclient-background-color: desaturate($o-gray-100, 100%);
+$o-control-panel-background-color: desaturate(#e9ecef, 100%); // $gray-200 bts4 default
 
 $o-list-footer-color: $o-main-text-color;
 $o-list-group-header-color: lighten($o-brand-lightsecondary, 10%);