chore: load 'assign_default' value from legacy value (#17428)

If this value was set before v2.19.0, then assign_default was in a json
field that would not match. And it would default to `false`. This
corrects that.

coderd/idpsync/organization.go

@@ -213,6 +213,17 @@ type OrganizationSyncSettings struct {
 }
 
 func (s *OrganizationSyncSettings) Set(v string) error {
+	legacyCheck := make(map[string]any)
+	err := json.Unmarshal([]byte(v), &legacyCheck)
+	if assign, ok := legacyCheck["AssignDefault"]; err == nil && ok {
+		// The legacy JSON key was 'AssignDefault' instead of 'assign_default'
+		// Set the default value from the legacy if it exists.
+		isBool, ok := assign.(bool)
+		if ok {
+			s.AssignDefault = isBool
+		}
+	}
+
 	return json.Unmarshal([]byte(v), s)
 }