Improve dropdown menus, remove inline styles (#24954)

Before:

<img width="190" alt="Screenshot 2023-05-27 at 10 46 43"
src="https://github.com/go-gitea/gitea/assets/115237/b9331fcd-db1d-476e-87f0-f79bae48b1a5">

After:

<img width="154" alt="Screenshot 2023-05-28 at 19 29 03"
src="https://github.com/go-gitea/gitea/assets/115237/8b7f99a2-01a8-4665-9342-a6201b51d30f">

---------

Co-authored-by: Giteabot <teabot@gitea.io>

templates/repo/issue/milestone/select_menu.tmpl

@@ -1,4 +1,3 @@
-<div class="header" style="text-transform: none;font-size:16px;">{{.locale.Tr "repo.issues.new.add_milestone_title"}}</div>
 {{if or .OpenMilestones .ClosedMilestones}}
 	<div class="ui icon search input">
 		<i class="icon gt-df gt-ac gt-jc">{{svg "octicon-search" 16}}</i>
@@ -8,7 +7,7 @@
 {{end}}
 <div class="no-select item">{{.locale.Tr "repo.issues.new.clear_milestone"}}</div>
 {{if and (not .OpenMilestones) (not .ClosedMilestones)}}
-	<div class="header" style="text-transform: none;font-size:14px;">
+	<div class="disabled item">
 		{{.locale.Tr "repo.issues.new.no_items"}}
 	</div>
 {{else}}