Fix K-12/new_styles Sass/CSS compilation

Fixes: CNVS-19013

Sass compilation in Canvas needs to be updated so
that the K-12 feature flag is no longer tied to the
use_new_styles feature flag.

Tiny change that simply sets $use_new_styles to
FALSE in the K-12 variant variables stylesheet.

QA Test Plan:

- Open Canvas in both K-12 and regular (legacy)
  UIs. Click around and make sure nothing has
  changed.

Change-Id: Iaa10357e9c780fa2a28ed9a443257d4ce9fefae9
Reviewed-on: https://gerrit.instructure.com/49767
Tested-by: Jenkins
Reviewed-by: Colleen Palmer <colleen@instructure.com>
Product-Review: Colleen Palmer <colleen@instructure.com>
Reviewed-by: Ryan Shaw <ryan@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>

Author: Chris Hart

app/stylesheets/variants/k12_high_contrast/_variant_variables.scss

@@ -1,3 +1,3 @@
-$use_new_styles: true;
+$use_new_styles: false;
 $is-k12: true;
 $use_high_contrast: true;

app/stylesheets/variants/k12_normal_contrast/_variant_variables.scss

@@ -1,3 +1,3 @@
-$use_new_styles: true;
+$use_new_styles: false;
 $is-k12: true;
 $use_high_contrast: false;