Rename default branch to main

[Francesco Lodolo]

This is one of the few repositories still using a master branch.

@pmac
As far as I've checked, it should be relatively quick to fix in Bedrock, and it wouldn't impact production.

Bedrock:

• Update LEGAL_DOCS_BRANCH setting

This repository:

• Create main branch
• Update references in GitHub Actions

[Paul McLanahan]

Sounds good. I can't think of anything else that would need updating apart from our local clones. Let's give it a go.

[Francesco Lodolo]

This should be done. On to the linter now (next week though).