ui: Reduce icons stroke width (#36361)

After redesigning all Zed icons
(https://github.com/zed-industries/zed/pull/35856), it felt like using
1.5 for stroke width didn't really flow well with the default typeface
default font weight. Reducing it to 1.2 makes the UI much sharper, less
burry, and more cohesive overall.

Release Notes:

- N/A

assets/icons/bell_off.svg

@@ -1,5 +1,5 @@
 <svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg">
-    <path d="M10.6667 5.8C10.6667 5.05739 10.3857 4.3452 9.88562 3.8201C9.38552 3.295 8.70724 3 8 3C7.29276 3 6.61448 3.295 6.11438 3.8201C5.61428 4.3452 5.33333 5.05739 5.33333 5.8C5.33333 9.06667 4 10 4 10H7.375" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
-    <path d="M6 12.5C6.19692 12.8028 6.48641 13.0554 6.83822 13.2313C7.19004 13.4072 7.59127 13.5 8 13.5C8.40873 13.5 8.80996 13.4072 9.16178 13.2313C9.51359 13.0554 9.80308 12.8028 10 12.5" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
-    <path d="M4 3L12.5 11.5" stroke="black" stroke-width="1.5" stroke-linecap="round"/>
+    <path d="M10.6667 5.8C10.6667 5.05739 10.3857 4.3452 9.88562 3.8201C9.38552 3.295 8.70724 3 8 3C7.29276 3 6.61448 3.295 6.11438 3.8201C5.61428 4.3452 5.33333 5.05739 5.33333 5.8C5.33333 9.06667 4 10 4 10H7.375" stroke="black" stroke-width="1.2" stroke-linecap="round" stroke-linejoin="round"/>
+    <path d="M6 12.5C6.19692 12.8028 6.48641 13.0554 6.83822 13.2313C7.19004 13.4072 7.59127 13.5 8 13.5C8.40873 13.5 8.80996 13.4072 9.16178 13.2313C9.51359 13.0554 9.80308 12.8028 10 12.5" stroke="black" stroke-width="1.2" stroke-linecap="round" stroke-linejoin="round"/>
+    <path d="M4 3L12.5 11.5" stroke="black" stroke-width="1.2" stroke-linecap="round"/>
 </svg>