fix(kanban): don't prevent setting a different bucket as done bucket

Resolves https://kolaente.dev/vikunja/frontend/issues/3373

pkg/models/kanban_test.go

@@ -217,7 +217,7 @@ func TestBucket_Update(t *testing.T) {
 
 		testAndAssertBucketUpdate(t, b, s)
 	})
-	t.Run("only one done bucket per project", func(t *testing.T) {
+	t.Run("old done bucket should be unset", func(t *testing.T) {
 		db.LoadAndAssertFixtures(t)
 		s := db.NewSession()
 		defer s.Close()
@@ -229,7 +229,16 @@ func TestBucket_Update(t *testing.T) {
 		}
 
 		err := b.Update(s, &user.User{ID: 1})
-		assert.Error(t, err)
-		assert.True(t, IsErrOnlyOneDoneBucketPerProject(err))
+		assert.NoError(t, err)
+		db.AssertExists(t, "buckets", map[string]interface{}{
+			"id":             1,
+			"project_id":     1,
+			"is_done_bucket": true,
+		}, false)
+		db.AssertExists(t, "buckets", map[string]interface{}{
+			"id":             3,
+			"project_id":     1,
+			"is_done_bucket": false,
+		}, false)
 	})
 }