update arm template to use newer api version for appsettings

[sjkp]

{
"apiVersion": "2015-08-01",
"name": "appsettings",
"type": "config",
"dependsOn": [
"[resourceId('Microsoft.Web/Sites', parameters('siteName'))]"
],
"properties": {
"SCM_SITEEXTENSIONS_FEED_URL": "https://www.siteextensions.net/api/v2/",
"letsencrypt:Hostnames": "[parameters('hostnames')]",
"letsencrypt:Email": "[parameters('email')]",
"letsencrypt:AcmeBaseUri": "https://acme-v01.api.letsencrypt.org/",
"letsencrypt:SubscriptionId": "[subscription().subscriptionId]",
"letsencrypt:Tenant": "[parameters('tenant')]",
"letsencrypt:ClientId": "[parameters('clientId')]",
"letsencrypt:ClientSecret": "[parameters('clientSecret')]",
"letsencrypt:ResourceGroupName": "[resourceGroup().name]"
}
},