[HEADS UP] Rename of the vendor/openzfs branch

Hello,

As mentioned in the "OpenZFS imports, status update":

    https://lists.freebsd.org/archives/freebsd-git/2021-June/000013.html

We're going to rename the current openzfs vendor branch,
vendor/openzfs, to vendor/openzfs/legacy
and import directly the master and zfs-2.1-release branches from the
upstream OpenZFS to vendor/openzfs/* for merging to our main and
stable/13 branches. The details can be found in the mail above.

The people have local branch tracking the original vendor/openzfs may
encounter error message like this whey doing `git pull`:

    error: cannot lock ref
'refs/remotes/freebsd/vendor/openzfs/legacy':
'refs/remotes/freebsd/vendor/openzfs' exists; cannot create
'refs/remotes/freebsd/vendor/openzfs/legacy'

The solution is update the upstream of the tracking branch:

    (change "freebsd" to "origin" if you use the default remote name)
    git remote prune freebsd
    (on your tracking branch)
    git branch -u freebsd/vendor/openzfs/legacy

If you don't want the first command prune all the branches not
existing in the remote repository, use this:

    git update-ref -d refs/remotes/freebsd/vendor/openzfs
    (also, change "freebsd" to "origin" if you use the default remote name)

After that you can do `git pull` or `git fetch` as usual.

If you have any questions, please post on -git@ list.

Best,
Li-Wen