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/settings/_JoinRuleSettings.pcss

@@ -27,7 +27,7 @@ limitations under the License.
 .mx_JoinRuleSettings_spacesWithAccess {
     > h4 {
         color: $secondary-content;
-        font-weight: $font-semi-bold;
+        font-weight: var(--font-semi-bold);
         font-size: $font-12px;
         line-height: $font-15px;
         text-transform: uppercase;
@@ -61,7 +61,7 @@ limitations under the License.
 
     .mx_StyledRadioButton_content {
         margin-left: 14px;
-        font-weight: $font-semi-bold;
+        font-weight: var(--font-semi-bold);
         font-size: $font-15px;
         line-height: $font-24px;
         color: $primary-content;