Clean up old branches?

[rsignell-usgs]

I was trying to see if there was a 4.8 branch with zarr, and was surprised to see so many branches and so many stale branches (some 5 years old) https://github.com/Unidata/netcdf-c/branches

[WardF]

This is something which needs doing for the sake of clarity if nothing else. I'll see if I'm able to devote some cycles to it in the next couple of weeks. Thanks!