Move the upstream-rebuild branch to main

[alexcrichton]

This was discussed a bit during the CG meeting yesterday but I wanted to open an issue for this as well. Downstream consumers such as the testsuite repository pick up changes to tests from the main branch, and additionally when visiting the repository it's useful to see the current state on the landing page, currently the main branch as well.

If I might offer a concrete suggestion, it might be useful to rename main to old-main (please bikeshed) and then rename the upstream-rebuild branch to main to avoid losing histories.