Revisit `::file-selector-button` prefixes and duplicate selector

The latest autoprefixer >= 10.3.0 supports prefixing ::file-selector-button.

Now, we need to eventually sort these issues:

1. we end up with a duplicate selector in reboot due to https://github.com/twbs/bootstrap/blob/ae16dbbb50bf4ffabe19fc71827b8b031d6141f4/scss/_reboot.scss#L576-L586 We have font: inherit in both selectors so we end up with this CSS:

::-webkit-file-upload-button {
  font: inherit;
}

::file-selector-button {
  font: inherit;
}

::-webkit-file-upload-button {
  font: inherit;
  -webkit-appearance: button;
}

2. we need to deprecate the prefixes in Sass and remove them in the next major version