Fix: notify locale on changed milestone email

What does this MR do and why?

Reason

Right now the email template for changed milestone notification can only be sent in English.

Result

Replace hardcoded raw string with i18n translations.

/cc @prajnamas @orozot @qk44077907

Steps to verify

1. Pull this branch.
2. Start your GDK.
3. Go to gdk/rails/mailers/notify/changed_milestone_merge_request_email for verify the changes.

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Before

After

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