Use `main` as primary branch

[lemeurherve]

trafficstars

Suggestion

We should use main instead of master as primary branch.

This will need updating not only this repo but also all references to master in Jenkinsfile(s) and job on ci.jenkins.io, infra.ci.jenkins.io and trusted.ci.jenkins.io

Links

• https://github.com/jenkins-infra/helpdesk/issues/2671