Rename default git branch `master` to `main`

[chalin]

In conformance to the inclusive naming initiatives of various organizations (including LF/CNCF, GH), master should be renamed to main. Note that it's been safe to do this for a while now given that GitHub handles the necessary redirects. For details, see:

• github/renaming
• Links to deleted branches now redirect to the default branch

Many (most?0 repos across LF/CNCF that have renamed their default branches.

/cc @nate-double-u

[varodrig]

I think we should do this.
cc
cc @kubeflow/kubeflow-steering-committee @kubeflow/wg-training-leads @kubeflow/wg-pipeline-leads @kubeflow/wg-notebooks-leads @kubeflow/wg-manifests-leads @kubeflow/release-team @kubeflow/wg-data-leads @kubeflow/wg-deployment-leads

[varodrig]

/area website

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