Add new site config sub-system intended to replace region (#13672)

- New sub-system has "region" and "name" fields.

- `region` subsystem is marked as deprecated, however still works, unless the
new region parameter under `site` is set - in this case, the region subsystem is
ignored. `region` subsystem is hidden from top-level help (i.e. from `mc admin
config set myminio`), but appears when specifically requested (i.e. with `mc
admin config set myminio region`).

- MINIO_REGION, MINIO_REGION_NAME are supported as legacy environment variables for server region.

- Adds MINIO_SITE_REGION as the current environment variable to configure the
server region and MINIO_SITE_NAME for the site name.

cmd/notification.go

@@ -63,7 +63,7 @@ func (sys *NotificationSys) GetARNList(onlyActive bool) []string {
 	if sys == nil {
 		return arns
 	}
-	region := globalServerRegion
+	region := globalSite.Region
 	for targetID, target := range sys.targetList.TargetMap() {
 		// httpclient target is part of ListenNotification
 		// which doesn't need to be listed as part of the ARN list
@@ -643,8 +643,8 @@ func (sys *NotificationSys) set(bucket BucketInfo, meta BucketMetadata) {
 	if config == nil {
 		return
 	}
-	config.SetRegion(globalServerRegion)
-	if err := config.Validate(globalServerRegion, globalNotificationSys.targetList); err != nil {
+	config.SetRegion(globalSite.Region)
+	if err := config.Validate(globalSite.Region, globalNotificationSys.targetList); err != nil {
 		if _, ok := err.(*event.ErrARNNotFound); !ok {
 			logger.LogIf(GlobalContext, err)
 		}