Replace Sass variables with custom properties on _font-weights.pcss (#10554)

* Replace `$font-normal` and `$font-semi-bold`

Signed-off-by: Suguru Hirahara <luixxiul@users.noreply.github.com>

* Replace `font-weight: 600` with the custom property

Signed-off-by: Suguru Hirahara <luixxiul@users.noreply.github.com>

* Replace the existing variables

Signed-off-by: Suguru Hirahara <luixxiul@users.noreply.github.com>

---------

Signed-off-by: Suguru Hirahara <luixxiul@users.noreply.github.com>

res/css/views/beta/_BetaCard.pcss

@@ -32,7 +32,7 @@ limitations under the License.
             flex: 1;
 
             .mx_BetaCard_title {
-                font-weight: $font-semi-bold;
+                font-weight: var(--font-semi-bold);
                 font-size: $font-18px;
                 line-height: $font-22px;
                 color: $primary-content;
@@ -126,7 +126,7 @@ limitations under the License.
     border-radius: 8px;
     text-transform: uppercase;
     font-size: $font-12px;
-    font-weight: $font-semi-bold;
+    font-weight: var(--font-semi-bold);
     line-height: 15px;
     color: $button-primary-fg-color;
     display: inline-block;