Migrate default branch from master to main

[vdemeester]

Last year, we talked a little bit about renaming the master branch to main for all the
tektoncd projects. So far, we change the organization settings to make all new repository
use main as default branch. The only two repository that are using main as default
branch are tektoncd/results and tektoncd/operator.

As promised last year, GitHub has delivered a feature to rename the master branch to main1 and since later last week, it is now finally available ! We can thus move on to our plan :).
According to the GitHub documentation, it will "redirect web requests for the old branch name to the new branch name". This means however, that any script, plumbing, tooling that is working on master might fail after renaming. So the plan is the following:

• work on making our scripts / plumbing more durable to allow both master and main (aka not making the assumption that the default branch is master)

• then go through our repos and rename things

• https://github.com/tektoncd/operator

• https://github.com/tektoncd/results

• https://github.com/tektoncd/friends

  • No problems

• https://github.com/tektoncd/homebrew-tools

  • homebrew impact ? no impact (and risk is very low)

• https://github.com/tektoncd/community

  • CI configuration
  • Tooling (?)
  • Links (TEPs, …)

• https://github.com/tektoncd/website

  • Netlify configuration
  • CI configuration
  • Tooling (?)

• https://github.com/tektoncd/plumbing

  • CI configuration
  • Tooling

• https://github.com/tektoncd/hub

  • CI configuration
  • Tooling (?)

• https://github.com/tektoncd/experimental

  • CI configuration
  • Tooling (?)

• https://github.com/tektoncd/chains

  • CI configuration
  • Tooling (?)

• https://github.com/tektoncd/dashboard

  • CI configuration
  • Releases scripts / pipelines
  • Tooling (?)

• https://github.com/tektoncd/cli

  • CI configuration
  • Releases scripts / pipelines
  • Tooling (?)

• https://github.com/tektoncd/triggers

  • CI configuration
  • Releases scripts / pipelines
  • Tooling (?)

• https://github.com/tektoncd/pipeline

  • CI configuration
  • Releases scripts / pipelines
  • Tooling (?)

• https://github.com/tektoncd/catalog

  • CI configuration
  • Releases scripts / pipelines
  • External links
    • Should be redirected but…
  • ArtifactHub ?

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For each repository, the process should go as follow:

• a PR is opened to make any tooling/script work with the main branch (as well as with
  master as much as we can)
• (once this PR is merged), we proceed to the branch renaming, using the GitHub feature
  (so that we get the redirection, changing PR base, …)…
• … and we update our CI setup (prow and/or tekton-based CI) to target/use main instead of
  master.
• we update any reference in our project to the migrated repository to use main instead of
  master (e.g. link to the community files).

Worst case, we "fail, learn and repeat" 😉.

The current proposed timeline is the following:

• 2020-01-27: migration of https://github.com/tektoncd/friends and https://github.com/tektoncd/homebrew-tools
• 2020-02-03: migration of https://github.com/tektoncd/community, https://github.com/tektoncd/website and https://github.com/tektoncd/plumbing
• 2020-03-03: migration of https://github.com/tektoncd/experimental, https://github.com/tektoncd/chains
  and https://github.com/tektoncd/dashboard
• 2020-03-10: migration of https://github.com/tektoncd/cli, https://github.com/tektoncd/triggers and https://github.com/tektoncd/pipeline
• 2020-03-17: migration of https://github.com/tektoncd/catalog and https://github.com/tektoncd/hub,

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/area plumbing
/kind misc
/assign

[bobcatfish]

Thanks for taking on this important work @vdemeester !!!!