Move alert theme-color() levels to variables

Closes #24341

scss/_alert.scss

@@ -46,6 +46,6 @@
 
 @each $color, $value in $theme-colors {
   .alert-#{$color} {
-    @include alert-variant(theme-color-level($color, -10), theme-color-level($color, -9), theme-color-level($color, 6));
+    @include alert-variant(theme-color-level($color, $alert-bg-level), theme-color-level($color, $alert-border-level), theme-color-level($color, $alert-color-level));
   }
 }

scss/_variables.scss

@@ -764,6 +764,10 @@ $alert-border-radius:               $border-radius !default;
 $alert-link-font-weight:            $font-weight-bold !default;
 $alert-border-width:                $border-width !default;
 
+$alert-bg-level:                    -10 !default;
+$alert-border-level:                -9 !default;
+$alert-color-level:                 6!default;
+
 
 // Progress bars