Rename references from master to main branch

[mrueg]

What this PR does / why we need it:
I've renamed the default branch to main according to #1554
How does this change affect the cardinality of KSM: (increases, decreases or does not change cardinality)
None
Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #1554

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@mrueg: This issue is currently awaiting triage.

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[fpetkovski]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: fpetkovski, mrueg

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