* lisp/erc/erc-dcc.el (erc-dcc-unquote-filename):
* lisp/erc/erc.el (erc-quit-reason-zippy, erc-part-reason-zippy)
(erc-update-mode-line-buffer, erc-message-english-PART): Use
'string-replace' only on Emacs 28 and later, otherwise use
'replace-regexp-in-string' on older Emacsen.