Fix markdown rendering in milestone content (#15056)

- 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

@@ -1167,6 +1167,7 @@
       > a {
         padding-top: 5px;
         padding-right: 10px;
+        font-size: 1.5rem;
         color: var(--color-text);
 
         &:hover {