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

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

This is a backport from BS5 to BS4.

As per https://github.com/twbs/bootstrap/issues/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.