update to .net 8

dotnet-sample/iac/main.bicep

@@ -1,7 +1,7 @@
 param webAppName string = uniqueString(resourceGroup().id) // Generate unique String for web app name
 param sku string = 'S1' // The SKU of App Service Plan
 param location string = resourceGroup().location // Location for all resources
-param linuxFxVersion string = 'DOTNETCORE|6.0' // The runtime stack of web app
+param linuxFxVersion string = 'DOTNETCORE|8.0' // The runtime stack of web app
 
 var appServicePlanName = toLower('AppServicePlan-gh-demo')
 var webSiteName = toLower('wapp-${webAppName}')