Fix the management portal url

[msJinLei]

• Fix the issue in Get-AzEnvironment and Get-AzContext return deprecated ManagementPortalUrl azure-powershell#15429
• Refer to the https://azure.microsoft.com/en-us/account/ for the official portal link (https://management.azure.com/metadata/endpoints?api-version=2019-05-01)

[msJinLei]

                if (string.IsNullOrEmpty(armMetadataRequestUri))
                {
                    armMetadataRequestUri = Environment.GetEnvironmentVariable(ArmMetadataEnvVariable);
                    if (string.IsNullOrEmpty(armMetadataRequestUri))
                    {
                        armMetadataRequestUri = DefaultArmMetaDataEndpoint;
                    }
                    else
                    {
                        debugLogger?.Invoke($"Get {armMetadataRequestUri} from environment variable {ArmMetadataEnvVariable}");
                    }
                }
                if (DisableArmMetaDataEndpoint.Equals(armMetadataRequestUri?.Trim().ToUpper()))
                {
                    warningLogger?.Invoke($"Discover feature is disabled by environment variable {ArmMetadataEnvVariable}");
                    armMetadataRequestUri = null;
                }
                if (!string.IsNullOrEmpty(armMetadataRequestUri))
                {

[isra-fel]

Looks good.
You might as well update the ManagementPortalUrl in example 3 of Get-AzEnvironment.md