Add hover outline to heatmap squares (#30828)

Makes it easier to use because you see which square is currently
hovered:

<img width="314" alt="Screenshot 2024-05-02 at 15 38 20"
src="https://github.com/go-gitea/gitea/assets/115237/3a15dad1-2259-4f28-9fae-5cf6ad3d8798">

I did try a `scoped` style for this, but that did not work for some
reason.

(cherry picked from commit 6f89d5e3a0886d02ead732005f593ae003f78f78)

web_src/css/features/heatmap.css

···
   padding: 0 5px;
 }
 
+#user-heatmap .vch__day__square:hover {
+  outline: 1.5px solid var(--color-text);
+}
+
 /* move the "? contributions in the last ? months" text from top to bottom */
 #user-heatmap .total-contributions {
   font-size: 11px;