Add compat from GNU ELPA as a soft dependency in ERC

Add compat from GNU ELPA as a soft dependency in ERC

* lisp/erc/erc-backend.el (erc-parse-server-response,
erc--parse-isupport-value): Remove sub-28 compat code involving
`string-search'.

* lisp/erc/erc-compat.el: Require compat package, but don't error
when absent.

* lisp/erc/erc-dcc.el (erc-dcc-member): Remove `string-search' compat
code.
(erc-dcc-unquote-filename): Remove `string-replace' compat code.

* lisp/erc/erc-speedbar.el (erc-speedbar-expand-server,
erc-speedbar-expand-channel, erc-speedbar-expand-user): Remove
`string-search' compat code.

* lisp/erc/erc.el: Add compat version 28.1.2.0 to Package-Requires
header and require `erc-compat' after other libraries.
(erc--valid-local-channel-p): Remove `string-search' compat code.
(erc-update-mode-line-buffer): Remove `string-replace' compat code.
(erc-message-english-PART): Remove `string-replace' compat code.