fix: remove deprecated 'template' provider

[jakemulley]

terraform-provider-template was deprecated by Hashicorp in October 2020 and hasn't had a release since. This means no binaries have ever been released for Apple Silicon, so if you have a Silicon device, you can't (or struggle to) run terraform init on Terraform configurations that use this module.

This replaces template_file usage with a generic replace() function for bucket policies as a workaround, as Terraform does not have an equivalent template() or template_string() function.