Yield to erc-move-to-prompt before unhiding prompt

Yield to erc-move-to-prompt before unhiding prompt

* lisp/erc/erc-backend.el (erc--hide-prompt): Change hook depth from 0
to 91 to allow the `move-to-prompt' module to do its thing.  This
feature was added by bug#54826 and first appeared in Emacs 29.
* lisp/erc/erc-common.el (erc-server-user): Remove erroneous comment.
The `buffers' field is a list of buffers.
* test/lisp/erc/erc-tests.el (erc-hide-prompt): Use `erc--target'
instead of `erc-default-recipients' because this is new code.