Add post-release action to run after releases

[haydenbaker]

Background

• Add a post-release workflow that runs after releases (or manually if needed)
  • This workflow can do various post-releases things, such as orchestrating updates to package-managers that vend the CLI (homebrew, scoop, etc.)
• Remove update-brew workflow, it is being added to the tap repo itself by Add autoupdate workflow homebrew-tap#23

Testing

• Using my personal forks (of smithy and homebrew-tap)
• Tested by invoking the post-release workflow here, and verifying it triggers the autoupdate workflow here, and the bottle-config gets updated here

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.