Part of umbrella issue kubernetes/org#2222

Issue to spec out and then run through renaming the default branch for this repository

• update pre/postsubmits to trigger on old branch name and new branch name
• determine whether periodics use default branch or have name hardcoded in git commands
  • if hardcoded, does attempting to checkout old name break?
• determine whether branch name is part of any job names
• determine if any docs need to be updated
• send notice
• rename branch (ref: https://github.com/github/renaming#renaming-existing-branches)
• if necessary, update periodics to refer to new branch name
• if necessary, rename jobs
• update branch protection config
• update jobs to only trigger on new branch name
• make sure things still work
  • keep an eye on ci and prs for a bit
  • trial the local development experience with pre-rename clone
• send notice

/sig contributor-experience
/area github-management
trialing a repo for this subproject
/wg k8s-infra
owns this repo
/sig testing
to discover any prow issues
/milestone v1.21
/priority important-soon
/assign @spiffxp
I plan on speccing this out, whether/when to execute depends on how kubernetes/org goes