Upgrade font awesome to 6.2.1

style/fa/_stacked.scss

@@ -1,4 +1,4 @@
-// Stacked Icons
+// stacking icons
 // -------------------------
 
 .#{$fa-css-prefix}-stack {
@@ -6,8 +6,8 @@
   height: 2em;
   line-height: 2em;
   position: relative;
-  vertical-align: middle;
-  width: ($fa-fw-width*2);
+  vertical-align: $fa-stack-vertical-align;
+  width: $fa-stack-width;
 }
 
 .#{$fa-css-prefix}-stack-1x,
@@ -16,6 +16,7 @@
   position: absolute;
   text-align: center;
   width: 100%;
+  z-index: var(--#{$fa-css-prefix}-stack-z-index, #{$fa-stack-z-index});
 }
 
 .#{$fa-css-prefix}-stack-1x {
@@ -27,5 +28,5 @@
 }
 
 .#{$fa-css-prefix}-inverse {
-  color: $fa-inverse;
+  color: var(--#{$fa-css-prefix}-inverse, #{$fa-inverse});
 }