Move to github.com/ko-build

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

- [x] create github.com/ko-build org
- [x] move this repo to github.com/ko-build (early September)
- [x] https://github.com/google/ko/pull/749
- [x] set up `ko.build` vanity importpath redirect using `` tag
- [x] #1025
- [x] `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!_
- [x] rename K8s slack channel `#ko-project` to `#ko-build` ([ref](https://github.com/kubernetes/community/compare/master...imjasonh:community-1:ko-build))
- [x] move https://github.com/imjasonh/setup-ko
- [x] update `setup-ko` to fetch released binaries from https://github.com/ko-build/ko/releases and `go install ko.build@main`
- [x] ~~cut v0.6~~ not needed yet
- GitHub will apparently redirect requests for `imjasonh/setup-ko`
- [x] https://github.com/chainguard-dev/terraform-provider-ko
- [x] 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!