Fix unexpected and inconsistent inheritance of line-height property for mx_TextualEvent (#8485)

- IRC layout: unspecified
- Modern layout: $font-22px (with _GroupLayout.scss)
- Bubble layout: $font-18px (with _EventBubbleTile.scss)

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

res/css/views/messages/_TextualEvent.scss

@@ -17,6 +17,7 @@ limitations under the License.
 .mx_TextualEvent {
     opacity: 0.5;
     overflow-y: hidden;
+    line-height: normal;
 
     a {
         color: $accent;