Fix markdown rendering in milestone content (#15056) (#15092)

- Add missing markdown class for rendered markdown.
- Increase font size of milestone name in list.

Fixes: https://github.com/go-gitea/gitea/issues/15046

web_src/less/_repository.less

@@ -1320,6 +1320,7 @@
         padding-top: 5px;
         padding-right: 10px;
         color: #000000;
+        font-size: 1.5rem;
 
         &:hover {
           color: #4078c0;