Update default development brain to `main`

[guidezpl]

trafficstars

There have been calls in the Git OSS community to replace "master" with a more inclusive alternative as the primary branch name. Core upstream committers have generally formed a consensus that the branch name for new repositories should be configurable, and that the default for this name should at some point be changed away from "master", most likely to "main".

Googlers, see go/pi-terms

[paulfthomas]

@dsn5ft please advise