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/admin-handlers.go

@@ -1466,7 +1466,7 @@ func getServerInfo(ctx context.Context, r *http.Request) madmin.InfoMessage {
 	return madmin.InfoMessage{
 		Mode:         string(mode),
 		Domain:       domain,
-		Region:       globalServerRegion,
+		Region:       globalSite.Region,
 		SQSARN:       globalNotificationSys.GetARNList(false),
 		DeploymentID: globalDeploymentID,
 		Buckets:      buckets,