attestation: specialize error on incompatible gh

[woodruffw]

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This is another attempt to iteratively improve the UX around operational attestation failures: it checks whether the failure is caused by a missing attestation subcommand, and directs the user to update gh as a resolution. This should be sound, since brew upgrade gh is exempt from the bootstrap cycle.

Example output:

$ brew install cowsay
==> Downloading https://formulae.brew.sh/api/formula.jws.json
###################################################### 100.0%
==> Downloading https://formulae.brew.sh/api/cask.jws.json
###################################################### 100.0%
Warning: cowsay has been deprecated because it has an archived upstream repository! It will be disabled on 2024-11-22.
==> Downloading https://ghcr.io/v2/homebrew/core/cowsay/manif
###################################################### 100.0%
==> Fetching cowsay
==> Downloading https://ghcr.io/v2/homebrew/core/cowsay/blobs
###################################################### 100.0%
==> Verifying attestation for cowsay
Error: The bottle for cowsay could not be verified.

This typically indicates an outdated or incompatible `gh` CLI.

Please confirm that you're running the latest version of `gh`
by performing an upgrade before retrying:

  brew update
  brew upgrade gh