Move to github.com/ko-build

An incomplete list of steps, more or less in order:

• create github.com/ko-build org
  • move this repo to github.com/ko-build (early September)
• https://github.com/google/ko/pull/749
  • set up ko.build vanity importpath redirect using <meta> tag
• #1025
• ghcr.io/google/ko image name change
  • ⚠️ GitHub will apparently not automatically redirect package requests; users will need to update their references to get future updates
• GitHub Releases download links will apparently redirect, so long as https://github.com/google/ko is not recreated later!
• rename K8s slack channel #ko-project to #ko-build (ref)
• move https://github.com/imjasonh/setup-ko
  • update setup-ko to fetch released binaries from https://github.com/ko-build/ko/releases and go install ko.build@main
  • cut v0.6 not needed yet
  • GitHub will apparently redirect requests for imjasonh/setup-ko
• https://github.com/chainguard-dev/terraform-provider-ko
  • update TF module release to point to new location, in new TF org

cc @mchmarny

If anybody thinks of anything that's missing, please let us know!