Older '.Net Framework' versions in Azure App Service may be open to a number of known and unknown vulnerabilities.
In Azure Console -
In Terraform -
References:
https://learn.microsoft.com/en-us/azure/app-service/configure-language-dotnet-framework
https://registry.terraform.io/providers/hashicorp/azurerm/latest/docs/resources/app_service#dotnet_framework_version