Avoid mutating default value of erc-server-last-peers

Avoid mutating default value of erc-server-last-peers

* lisp/erc/erc-backend.el (erc-server-last-peers): Leave default as
nil instead of a quoted constant.
(erc-server-connect): Initialize `erc-server-last-peers' to a new
value local to a server buffer.
(erc-message): Operate on server's local `erc-server-last-peers' value
instead of the global default.  Prefer replacing value instead of
mutating CDR to make for easier testing.
(erc-server-PRIVMSG): Create a new `erc-server-last-peers' for easier
testing. (Bug#56449)