Fix config leaks and deprecate file-based config setters in NAS gateway (#9884)

This PR has the following changes

- Removing duplicate lookupConfigs() calls.
- Deprecate admin config APIs for NAS gateways. This will avoid repeated reloads of the config from the disk.
- WatchConfigNASDisk will be removed
- Migration guide for NAS gateways users to migrate to ENV settings.

NOTE: THIS PR HAS A BREAKING CHANGE

Fixes #9875

Co-authored-by: Harshavardhana <harsha@minio.io>

cmd/config.go

@@ -23,7 +23,6 @@ import (
 	"path"
 	"sort"
 	"strings"
-	"time"
 
 	jsoniter "github.com/json-iterator/go"
 	"github.com/minio/minio/cmd/config"
@@ -184,23 +183,6 @@ func (sys *ConfigSys) Load(objAPI ObjectLayer) error {
 	return sys.Init(objAPI)
 }
 
-// WatchConfigNASDisk - watches nas disk on periodic basis.
-func (sys *ConfigSys) WatchConfigNASDisk(ctx context.Context, objAPI ObjectLayer) {
-	configInterval := globalRefreshIAMInterval
-	watchDisk := func() {
-		for {
-			select {
-			case <-ctx.Done():
-				return
-			case <-time.After(configInterval):
-				loadConfig(objAPI)
-			}
-		}
-	}
-	// Refresh configSys in background for NAS gateway.
-	go watchDisk()
-}
-
 // Init - initializes config system from config.json.
 func (sys *ConfigSys) Init(objAPI ObjectLayer) error {
 	if objAPI == nil {