v0.20.0 Pin GitHub provider to 3.0.0, Last Terraform 0.12 support

• Last Terraform 0.12 support
• Pin GitHub provider to 3.0.0 for bug fixes
• Remove github_webhooks_anonymous no longer supported by GitHub and causing cloudposse/terraform-aws-ecs-web-app#63

Note that this version is affected by a bug that puts attributes provided by the caller in the wrong place in some identifiers. If you are not providing attributes, it is not an issue, but if you are, please upgrade instead to v0.21.0 which has the correct order. Otherwise you may find resources being unnecessarily deleted.

Pin GitHub provider to 3.0.0 @Nuru (#30)

what

• Restore TF 0.12 support
• Remove github_anonymous
• Use modules pinned to GitHub provider 3.0.0

why

• Update/fix TF 0.12 compatible module before dropping support for TF 0.12
• GitHub provider dropped support for github_anonymous
• GitHub provider has breaking changes with every release, 3.0.0 seemed the best balance of stable, bug-free, and compatible with our codebase