K3s release improvements

[tashima42]

Changes:

• Closes: Failed to create RC k3s release #323 adds the missing k3s-io remote to create the PR against.
• Closes: Add update go version to modify k3s #322 the go version wasn't being updated in the modify command, this was added.
• Adds macOS support
• Adds dry run for the modify script
• Renames OldK3SVersion and NewK3SVersion to OldK3SSuffix and NewK3SSuffix to reflect their usage in the code
• Renames Token to GithubToken
• Moves GithubToken out of the config to an env var
• Adds some customization parameters to run the commands against forks instead of k3s-io

How to test

• Fork tashima42/k3s and tashima42/kubernetes copying all tags, not just master
  Config:

{
  "old_k8s_version": "v1.27.4",
  "new_k8s_version": "v1.27.5",
  "old_k8s_client": "v0.27.4",
  "new_k8s_client": "v0.27.5",
  "old_k3s_suffix": "k3s1",
  "new_k3s_suffix": "k3s1",
  "release_branch": "release-1.27",
  "workspace": "$GOPATH/src/github.com/kubernetes",
  "handler": "YOURHANDLER",
  "email": "[email protected]",
  "ssh_key_path": "$HOME/.ssh/id_ed25519",
  "k3s_remote": "YOURHANDLER",
  "k8s_rancher_url": "[email protected]:YOURHANDLER/kubernetes.git",
  "k3s_upstream_url": "https://github.com/YOURHANDLER/k3s",
  "dry_run": true
}

cat config-template.json | envsubst > config.json
export GITHUB_TOKEN=your_token
k3s_release create-tags -c config.json
k3s_release push-tags -c config.json
k3s_release modify-k3s -c config.json
k3s_release tag-rc-release -c config.json
k3s_release tag-release -c config.json