🐛 Fixe a keystring misuse and refactor duplicates keystrings (#32668) (#32792)

Backport #32668 

- Fixes a translation keystring misuse where the string 'open
milestones' is used in place of 'closed milestones'.
- De-duplicates the use of 'open milesones' and 'closed milestones'
keystrings on the sidebar of an issue, reusing the ones on the issues
filter and action bars.
- Closes #32667

Co-authored-by: Simon Pistache <105607989+SimonPistache@users.noreply.github.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/repo/issue/filter_actions.tmpl

@@ -58,7 +58,7 @@
 				{{end}}
 				{{if .ClosedMilestones}}
 					<div class="divider"></div>
-					<div class="header">{{ctx.Locale.Tr "repo.issues.filter_milestone_open"}}</div>
+					<div class="header">{{ctx.Locale.Tr "repo.issues.filter_milestone_closed"}}</div>
 					{{range .ClosedMilestones}}
 						<div class="item issue-action" data-element-id="{{.ID}}" data-url="{{$.RepoLink}}/issues/milestone">
 							{{.Name}}

templates/repo/issue/milestone/select_menu.tmpl

@@ -14,7 +14,7 @@
 	{{if .OpenMilestones}}
 		<div class="divider"></div>
 		<div class="header">
-			{{ctx.Locale.Tr "repo.issues.new.open_milestone"}}
+			{{ctx.Locale.Tr "repo.issues.filter_milestone_open"}}
 		</div>
 		{{range .OpenMilestones}}
 			<a class="item" data-id="{{.ID}}" data-href="{{$.RepoLink}}/issues?milestone={{.ID}}">
@@ -26,7 +26,7 @@
 	{{if .ClosedMilestones}}
 		<div class="divider"></div>
 		<div class="header">
-			{{ctx.Locale.Tr "repo.issues.new.closed_milestone"}}
+			{{ctx.Locale.Tr "repo.issues.filter_milestone_closed"}}
 		</div>
 		{{range .ClosedMilestones}}
 			<a class="item" data-id="{{.ID}}" data-href="{{$.RepoLink}}/issues?milestone={{.ID}}">