Updated the small-font-size to use a round value

This is a backport from BS5 to BS4.

As per #35553

The ```small-font -size``` will actually increase from ```13px``` to ```14px``` with this proposed change. Personally I'd prefer keeping it the BS5 way (```14px```), as the text is more readable now.

If it's needed, I can do another PR with the ```$small-font-size:             .8125rem !default;``` to resemble BS4 more closely but I feel like this would be a move forward even for BS4.

scss/_variables.scss

@@ -319,7 +319,7 @@ $display-line-height: $headings-line-height !default;
 $lead-font-size: $font-size-base * 1.25 !default;
 $lead-font-weight: 300 !default;
 
-$small-font-size: 80% !default;
+$small-font-size: .875em !default;
 
 $text-muted: $gray-600 !default;