style: Scale calendar heading with font size

Using a fixed pixel size can result in the cell being too small for the
contained label. Instead use em to scale with the font size like is done
for the other cells. The chosen size corresponds to 16px with the
default font settings.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/6651
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2759>

data/theme/gnome-shell-sass/widgets/_calendar.scss

@@ -94,7 +94,7 @@
     &.calendar-day-heading {
       color: $insensitive_fg_color;
       padding-top: $base_padding;
-      height: 16px !important; // force heading to be smaller height
+      height: 1.1em !important; // force heading to be smaller height
       font-weight: 600;
       @extend %smaller;
     }