attestation: improve error message when gh is too old

[nandahkrishna]

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Currently, when gh is too old to verify attestations, the error message is uninterpretable:

==> Installing frizbee dependency: go
==> Downloading https://ghcr.io/v2/homebrew/core/go/manifests/1.22.5
==> Verifying attestation for go
Error: The bottle for go has an invalid build provenance attestation.

This may indicate that the bottle was not produced by the expected
tap, or was maliciously inserted into the expected tap's bottle
storage.

Additional context:

attestation verification failed: Failure while executing; `/usr/bin/env GH_TOKEN=****** /opt/homebrew/bin/gh attestation verify /Users/REDACTED/Library/Caches/Homebrew/downloads/3f2258381d80802127c7c7d253acee16b65864703253acd14e6e63e6194b0b62--go--1.22.5.arm64_sonoma.bottle.tar.gz --repo trailofbits/homebrew-brew-verify --format json` exited with 1. Here's the output:
Failed to verify the artifact: failed to fetch attestations for subject: sha256:555bf0e72c91ff434d4ab9e69c001cd998f6ce23fdddcac7013f94343d0defda

This PR aims to improve the messaging, like below:

➜  homebrew git:(attestation-gh-error) ✗ HOMEBREW_NO_INSTALL_FROM_API=1 brew install -sdv frizbee
[TRUNCATED]
==> Installing dependencies for frizbee: go
==> Installing frizbee dependency: go
==> Downloading https://ghcr.io/v2/homebrew/core/go/manifests/1.22.5
==> Verifying attestation for go
Error: `gh` is too old, you must run `brew upgrade gh` to continue.

[MikeMcQuaid]

One note then good to 🚢!

[nandahkrishna]

I've ignored bootstrapping the latest gh or trying to upgrade it because it's a little too complicated for what it's worth. Now this just raises an error if gh is too old, and asks the user to upgrade it.