[Kobzol]

This one is a bit special, because https://github.com/docker-library/official-images reads data from this repository from the master branch by default (from this file).

The second commit in this PR updates the config for that repo to read from the main branch itself (modeled after e.g. dart), but that change won't be propagated immediately.

So the rename on this repo should work as follows:

• Rename the master branch on this repository to main (as usually)
• (ideally immediately after) Push a copy of the renamed main branch to a master branch on this repo (which will now be non-default), so that official-images still continue working

Once official-images is updated, we can remove the master branch. Note that it is important to actually rename the master branch to main, rather than just create a new main branch, so redirects and the GitHub rename automation works as expected. Although I suspect that once we create a new master branch, we will break the redirect links anyway.. But I'm not sure if that's super important. In theory we could also just not do the second step and just go update official-images. I don't know if the update there is immediate, or if some old stuff might break.