Bury new ERC buffers by default

Bury new ERC buffers by default

* lisp/erc/erc.el (erc-join-buffer): Change default value to `bury'.
(erc-setup-buffer): Make `window-noselect' behave more like its
description and abstain from ever replacing the current buffer.
* test/lisp/erc/erc-scenarios-base-reconnect.el
(erc-scenarios-common-base-reconnect-options): Update helper to handle
new default value for option `erc-join-buffer'.
(erc-scenarios-base-reconnect-options--buffer): Update and rename
function `erc-scenarios-base-reconnect-options--default'.
(erc-scenarios-base-reconnect-options--default): Update and rename
function `erc-scenarios-base-reconnect-options--bury'.
* etc/ERC-NEWS: Update existing display-buffers section for 5.5.
(Bug#51753)