Migrate mattes/migrate to new repo golang-migrate/migrate

[djui]

The mattes/migrate repo is archived since 8th May and superseded by golang-migrate/migrate now. Support for this repo instead (or mid-term both side-by-side to not break current app deployments) would be nice to keep up with features and security patches (v3.0.1 vs v.3.4.0).

Of course, one can fork the buildpack or use install it as a vendored go get dependency, but would be nice to maintain that feature.

I might give the PR a try, but seeing how the repo patch is regexp in different tools (sed, grep, jq, tq) will likely make supporting both more complex that the benefit it offers.

[djui]

Looking a bit closer, I actually believe this feature never worked, as the release binary filename (at least nowadays is migrate.linux-amd64.tar.gz not migrate-linux-amd64.tar.gz, which should have caused any in-the-wild users to raise issues.

So maybe a breaking change (and only supporting the new repo) is tolerable here.

[djui]

Looking a bit further into the code, the additional-tools only supports migrate currently, which is not obvious from the README and support article. Maybe simplifying that feature and making it more flexible by letting its value be a script (as file or command[s]) in the repo to be run, is easier.

For example (and for compatibility reasons likely with a different key script|additional-script:

[metadata.heroku]
  additional-tools = ["./scripts/download_migrate.sh"]

or

[metadata.heroku]
  additional-tools = [
    [
      "curl -s -f -O migrate.tar.gz -L https://github.com/golang-migrate/migrate/releases/download/v3.4.0/migrate.linux-amd64.tar.gz", 
      "tar xvz migrate.tar.gz",
    ]
  ]

Anyway, whole different can of worms.