Clean up diff header css and reduce global textarea min-height (#29232)

1. Tweak diff header and remove a numbe of unneeded CSS for it:

Before:
<img width="433" alt="Screenshot 2024-02-18 at 01 08 09"
src="https://github.com/go-gitea/gitea/assets/115237/d8b377c0-57bc-44d5-bb57-a582c7d4b3b4">

After:
<img width="463" alt="Screenshot 2024-02-18 at 01 07 56"
src="https://github.com/go-gitea/gitea/assets/115237/d08c17e7-5b86-4d07-81da-6371f4754325">

3. Reduce height of review textarea and also reduce fomantic's CSS from
12em to 8em. Now fits better on my screen:

<img width="1352" alt="image"
src="https://github.com/go-gitea/gitea/assets/115237/5c658d13-295e-4929-94da-13ade888020d">

---------

Co-authored-by: delvh <dev.lh@web.de>

web_src/css/base.css

@@ -21,6 +21,7 @@
   --border-radius-circle: 50%;
   --opacity-disabled: 0.55;
   --height-loading: 16rem;
+  --min-height-textarea: 132px; /* padding + 6 lines + border = calc(1.57142em + 6lh + 2px), but lh is not fully supported */
   --tab-size: 4;
 }
 
@@ -492,6 +493,11 @@ ol.ui.list li,
   background: var(--color-active) !important;
 }
 
+.ui.form textarea:not([rows]) {
+  height: var(--min-height-textarea); /* override fomantic default 12em */
+  min-height: var(--min-height-textarea); /* override fomantic default 8em */
+}
+
 /* styles from removed fomantic transition module */
 .hidden.transition {
   visibility: hidden;