Add missing 'email_due_date' language string

Commit 0fd3ba1 referenced a new string
but did not define it.

Fixes #20806

lang/strings_english.txt

@@ -337,6 +337,7 @@ $s_email_description = 'Description';
 $s_email_additional_information = 'Additional Information';
 $s_email_steps_to_reproduce = 'Steps to Reproduce';
 $s_email_tag = 'Tags';
+$s_email_due_date = 'Due Date';
 
 # account_delete.php
 $s_account_protected_msg = 'Account protected. Cannot change settings...';