Merge branch 'remove-scss-prefix-linear-gradient' into 'master'

Remove vendor prefixes for linear-gradient CSS

## What does this MR do?
Removes the vendor prefixes for linear-gradient and the linear-gradient mixin since they are no longer needed with our latest and greatest browser (as per [annabeldunstone](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5762#note_14353141))  

## Are there points in the code the reviewer needs to double check?
None

## Why was this MR needed?
Reduces unnecessary CSS

## Screenshots (if relevant)
None

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?
*  Closes #21558

See merge request !6063

CHANGELOG

@@ -3,6 +3,7 @@ Please view this file on the master branch, on stable branches it's out of date.
 v 8.12.0 (unreleased)
   - Make push events have equal vertical spacing.
   - Add two-factor recovery endpoint to internal API !5510
+  - Remove vendor prefixes for linear-gradient CSS (ClemMakesApps)
   - Add font color contrast to external label in admin area (ClemMakesApps)
   - Change logo animation to CSS (ClemMakesApps)
   - Change merge_error column from string to text type

app/assets/stylesheets/framework/nav.scss

@@ -8,10 +8,7 @@
   height: 30px;
   transition-duration: .3s;
   -webkit-transform: translateZ(0);
-  background: -webkit-linear-gradient($gradient-direction, rgba($gradient-color, 0.4), $gradient-color 45%);
-  background: -o-linear-gradient($gradient-direction, rgba($gradient-color, 0.4), $gradient-color 45%);
-  background: -moz-linear-gradient($gradient-direction, rgba($gradient-color, 0.4), $gradient-color 45%);
-  background: linear-gradient($gradient-direction, rgba($gradient-color, 0.4), $gradient-color 45%);
+  background: linear-gradient(to $gradient-direction, $gradient-color 45%, rgba($gradient-color, 0.4));
 
   &.scrolling {
     visibility: visible;