Rename branch from `master` to `main`

[Emily Jiang]

In preparing for MicroProfile 6.1 release, for the updated specs, I would like to do the branch rename exercise: master -> main for the specs: config, metrics. Telemetry has already got the main branch.

With this exercise, I will list what are needed to be updated so that the other specs can prepare for their branch rename. If you have any objections or concerns, please shout now.

Thanks,

Emily