fix(#32667): 🐛 Fixe a keystring misuse and refactor duplicates keystrings (#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

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Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

options/locale/locale_en-US.ini

@@ -1461,8 +1461,6 @@ issues.new.no_items = No items
 issues.new.milestone = Milestone
 issues.new.no_milestone = No Milestone
 issues.new.clear_milestone = Clear milestone
-issues.new.open_milestone = Open Milestones
-issues.new.closed_milestone = Closed Milestones
 issues.new.assignees = Assignees
 issues.new.clear_assignees = Clear assignees
 issues.new.no_assignees = No Assignees