[Next Theme] Revert font-weight of OuiButton to normal from semi-bo…

…ld (#1222) (#1227)

* The larger font-size eliminates the need for bolder text

Signed-off-by: Miki <miki@amazon.com>
(cherry picked from commit 53219be)
Signed-off-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

# Conflicts:
#	CHANGELOG.md

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

src/themes/oui-next/global_styling/mixins/_button.scss

@@ -51,7 +51,7 @@
 
   text-decoration: none;
   border: solid 1px transparent;
-  font-weight: 600;
+  font-weight: 400;
 
   // sass-lint:disable mixins-before-declarations
   // focus states should come after all default styles