DEPRECATION Warning: Avoid configuring default LDAP STS Expiry (#12781)

- Show notice when `MINIO_IDENTITY_LDAP_STS_EXPIRY` or the 
  corresponding to the configuration option is used at server startup.
- Once support is removed, the default will be fixed at 1 hour.
- Users may specify expiry directly in the STS API.
- Update docs and help message
- Adds example in ldap.go to configure expiry in STS API.

internal/config/identity/ldap/help.go

@@ -30,7 +30,7 @@ var (
 		},
 		config.HelpKV{
 			Key:         STSExpiry,
-			Description: `temporary credentials validity duration in s,m,h,d. Default is "1h"`,
+			Description: `[DEPRECATED] temporary credentials validity duration in s,m,h,d. Default is "1h"`,
 			Optional:    true,
 			Type:        "duration",
 		},