Move "master" to "main" branch.

[bwplotka]

Not trivial if we want to do it properly e.g. change docker image tag: https://hub.docker.com/r/prominfra/prombench/tags

If we want, at minimum:

• we need change our CI to be triggered on right branch https://github.com/prometheus/test-infra/blob/master/.circleci/config.yml#L145 (and so on)
• We need right branch to be exposed https://github.com/prometheus/test-infra/blob/master/.circleci/config.yml#L54 ( and so on)
• Sync Prometheus use of images https://github.com/prometheus/prometheus/blob/main/.github/workflows/prombench.yml#L34

(Maybe more, to be tested).

[bwplotka]

~Also https://github.com/prometheus/test-infra/blob/master/.github/dependabot.yml is not working correctly now - it assumes "main". (When I tmp change master to main, only then we got bunch of upgrade PRs)~ Ignore this, it's was incidental, all good.