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

templates/repo/issue/milestone_issues.tmpl

@@ -4,8 +4,8 @@
 	<div class="ui container">
 		<div class="ui three column stackable grid">
 			<div class="column">
-				<h3>{{.Milestone.Name}}</h3>
-				<div class="content">
+				<h1>{{.Milestone.Name}}</h1>
+				<div class="markdown content">
 					{{.Milestone.RenderedContent|Str2html}}
 				</div>
 			</div>