Use json to fetch latest versions

[SuperQ]

Go provides a json url to fetch the latest versions: https://go.dev/dl/?mode=json

We should use this data rather than polling for file download URLs.

[gitperr]

How is it being polled at the moment? I thought it was being hardcoded.

Can you extend this example a bit? I can try to implement it.

[SuperQ]

The builder-bumper does a really basic HTTP HEAD request on the next possible version(s) to see if the release tar files exist to download.

[gitperr]

Are we only interested in linux-amd ones?

I have some dirty code at the moment that fetches the link for it, but I could iterate more and add some other things too.

$ ./builder-bumper "1.20" "1.21"
https://dl.google.com/go/go1.22.0.linux-amd64.tar.gz

[SuperQ]

We only actually care about the current stable version numbers, so we can decide if we need to update.

[gitperr]

Should it return the version(s) only instead?

[SuperQ]

Please take a look at what the builder-bumper does, the actual functionality should not change. Just how it detects what version(s) to update.