fisher self-update no longer exists as of 4.0.0

[OkanEsen]

What did you expect to happen?

Fisher should update itself and the installed plugins.

What actually happened?

The upgrade step throws an error, since fisher self-update is no longer available.

Additional details:

According to the maintainer of fisher, as of 4.0.0, the user is expected to update fisher by installing itself:

We no longer need self-update and self-uninstall either since we can just:

fisher update fisher

or to update everything (including fisher):

fisher update

See: jorgebucaran/fisher#594