Move sprint error locales to backlogs module

Moves `must_be_in_planning` and `only_one_active_sprint_allowed`
from core `config/locales/en.yml` to the backlogs module's
locale file, as these are sprint-specific error messages.

Author: myabc

config/locales/en.yml

@@ -2002,8 +2002,6 @@ en:
         not_editable: "cannot be edited because it is already in effect."
         not_current_user: "is not the current user."
         system_wide_non_working_day_exists: "conflicts with an existing system-wide non-working day for this date."
-        must_be_in_planning: "must be in planning to start."
-        only_one_active_sprint_allowed: "only one active sprint is allowed per project."
         not_found: "not found."
         not_a_date: "is not a valid date."
         not_a_datetime: "is not a valid date time."

modules/backlogs/config/locales/en.yml

@@ -32,6 +32,12 @@ en:
     name: "OpenProject Backlogs"
     description: "This module adds features enabling agile teams to work with OpenProject in Scrum projects."
 
+  activemodel:
+    errors:
+      messages:
+        must_be_in_planning: "must be in planning to start."
+        only_one_active_sprint_allowed: "only one active sprint is allowed per project."
+
   activerecord:
     attributes:
       agile/sprint: