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/file_icons/conversations.svg

@@ -1,4 +1,4 @@
 <svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg">
 <path d="M6.46115 9.43419C8.30678 9.43419 9.92229 8.43411 9.92229 6.21171C9.92229 3.98933 8.30678 2.98926 6.46115 2.98926C4.61553 2.98926 3 3.98933 3 6.21171C3 7.028 3.21794 7.67935 3.58519 8.17685C3.7184 8.35732 3.69033 8.77795 3.58387 8.97539C3.32908 9.44793 3.81048 9.9657 4.33372 9.84571C4.72539 9.75597 5.13621 9.63447 5.49574 9.4715C5.62736 9.41181 5.7727 9.38777 5.91631 9.40402C6.09471 9.42416 6.27678 9.43419 6.46115 9.43419Z" fill="black" stroke="black" stroke-linecap="round" stroke-linejoin="round"/>
-<path d="M12.3385 7.24835C12.7049 7.74561 12.9224 8.39641 12.9224 9.2117C12.9224 10.028 12.7044 10.6793 12.3372 11.1768C12.204 11.3573 12.232 11.7779 12.3385 11.9754C12.5933 12.4479 12.1119 12.9657 11.5886 12.8457C11.197 12.756 10.7862 12.6345 10.4266 12.4715C10.295 12.4118 10.1497 12.3878 10.0061 12.404C9.82765 12.4242 9.64558 12.4342 9.46121 12.4342C8.61469 12.4342 7.81658 12.2238 7.20055 11.7816" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+<path d="M12.3385 7.24835C12.7049 7.74561 12.9224 8.39641 12.9224 9.2117C12.9224 10.028 12.7044 10.6793 12.3372 11.1768C12.204 11.3573 12.232 11.7779 12.3385 11.9754C12.5933 12.4479 12.1119 12.9657 11.5886 12.8457C11.197 12.756 10.7862 12.6345 10.4266 12.4715C10.295 12.4118 10.1497 12.3878 10.0061 12.404C9.82765 12.4242 9.64558 12.4342 9.46121 12.4342C8.61469 12.4342 7.81658 12.2238 7.20055 11.7816" stroke="black" stroke-width="1.2" stroke-linecap="round" stroke-linejoin="round"/>
 </svg>