update master references to main

[marc-vdm]

Question for @mrvisscher : should we actually move this file to minor instead (and rename the file)?

Checklist

• Keep pull requests small so they can be easily reviewed.
• Update the documentation, please follow the numpy style guide.
• Update tests.
• Categorize the PR by setting a good title and adding one of the labels:
  bug, feature, ui, change, documentation, breaking, ci
  as they show up in the changelog.
• Link this PR to related issues by using closing keywords.
• Add a milestone to the PR (and related issues, if any) for the intended release.
• Request a review from another developer.