feat(stepper): enable responsive layout (#7744)

**Related Issue:** #6694

Enables responsive layout for stepper component.

- stepper element `host` display is changed to `flex` from `grid`
- grid column width will use `minmax( )` instead of fixed fractional
units.
- When none of the stepper item's are selected and the first one is
disabled, the first available enabled stepper will be selected.

packages/calcite-components/src/components/stepper/stepper.scss

@@ -86,7 +86,7 @@
   }
 
   .step-bar--inactive {
-    fill: var(--calcite-ui-border-3, #dfdfdf);
+    fill: var(--calcite-color-border-3, #dfdfdf);
     fill-opacity: 1;
     block-size: 100%;
   }