Dated terraform provider for fastly

[miketheman]

We're currently using fastly provider 1.1.2 and latest release is 5.2.2
See changelog: https://github.com/fastly/terraform-provider-fastly/blob/main/CHANGELOG.md

[shenxianpeng]

Now that the last version of fastly is 5.8.0, it might be good to add dependabot.yml to identify which dependencies are outdated and then consider upgrading. I can send a pull request if needed.