[FIX] web: set the right text-muted from rgba value for o-bg-color mixin

Correctly generate a text-muted color starting from an rgba value.
The 'scale-color' function will take in account the source color's
alpha, decreasing its value.

e.g.
```
$color: rgba(#fff, 0.7);

$muted: rgba($-yiq-color, 0.7);
  -> rgba(#fff, 0.7) (same as source value)

$muted: scale-color($-yiq-color, $alpha: -30%);
  -> rgba(#fff, 0.42) (correct)
```

Part of https://github.com/odoo/odoo/pull/45856
task-2197038

addons/web/static/src/scss/bs_mixins_overrides.scss

@@ -22,7 +22,7 @@ $enable-deprecation-messages: false !default;
 
         @if ($with-muted) {
             .text-muted {
-                color: rgba($-yiq-color, 0.4) !important;
+                color: scale-color($-yiq-color, $alpha: -30%) !important;
             }
         }