Enable smooth scrolling in Safari

Context

The following discussion from gitlab-ui!2067 (merged) should be addressed:

• @pslaughter started a discussion: (+2 comments)

  issue (ux, safari): Unfortunately, it looks like the scrollTo with options is not supported by Safari (see MDN), which I believe is one of our support browsers...

  20210331_scrollable_tabs_safari

  WDYT @jareko & @fguibert? You'll also notice the gradient blur isn't quite working the way we expect here 😞

  @fguibert, if needed we might also have a smooth scroll function in GitLab which has some polyfilled support 🤔

Our list of supported browsers changed in %15.2: https://docs.gitlab.com/ee/update/removals.html#support-for-older-browsers. So it looks like we only need to worry about Safari >=14.1.