Remove BrewTestBot autocommits

[MikeMcQuaid]

Currently update-database.yml is run on a schedule which automatically commits changes to executables.txt.

Instead, we should move this logic into https://github.com/Homebrew/formulae.brew.sh and consider using the path_exec_files from the bottle manifest rather than needing to download the bottles for this.

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Edits from @Rylan12:

Steps:

• Start pushing executables.txt to GitHub Packages
  • Push executables.txt to GitHub Packages #248
• Serve executables.txt on formulae.brew.sh
  • Add executables.txt from Homebrew/command-not-found to internal API formulae.brew.sh#1936
• Modify brew which-formula to download from formulae.brew.sh instead of the local file
• Convert executables.txt to a JSON file
• Stop auto-committing and remove executables.txt from this repository
• Keep track of executables during bottle creation, and store in OCI manifest file
• Update executables.txt in formulae.brew.sh by downloading the OCI manifest files and pulling the executable list from there
• Remove brew which-update and all automated jobs in this repo
• Migrate brew which-formula to Homebrew/brew