For new go toolchain releases, send a webhook only once, not for each platform instance of the toolchain

[mjl-]

We're now doing many webhook calls, and in quick succession, we may even get rate limited. There's only need for one webhook call.

[mjl-]

Or perhaps better: Change webhook format so we can send multiple module/versions in a single call. We can send them all, and the recipient can look for the platforms (goos/goarch) they are interested in.