Update z-indices (#29292)

- `.carousel-caption` doesn't need a z-index. There are no non-static element in its containing block (`.carousel-item`)
- The `z-index` of `.carousel-indicators` is way higher than it needs to be (`15`), the highest sibling `z-index` is `1`, so it can be set to `2` to be the top layer.

scss/_carousel.scss

@@ -148,7 +148,7 @@
   right: 0;
   bottom: 0;
   left: 0;
-  z-index: 15;
+  z-index: 2;
   display: flex;
   justify-content: center;
   padding-left: 0; // override <ol> default
@@ -190,7 +190,6 @@
   right: (100% - $carousel-caption-width) / 2;
   bottom: 20px;
   left: (100% - $carousel-caption-width) / 2;
-  z-index: 10;
   padding-top: 20px;
   padding-bottom: 20px;
   color: $carousel-caption-color;