Deprecate erc-button-nickname-callback-function

Deprecate erc-button-nickname-callback-function

* lisp/erc/erc-button.el (erc-button-nickname-callback-function):
Deprecate this function-valued variable, first introduced in ERC 5.6
and Emacs 30, to dissuade consumers of the old `erc-button-alist'
nickname interface from meddling with the on-click callback of
buttonized nicks.  If necessary, third parties can instead add their
own propertizing logic in something like `erc-insert-modify-hook'.
Also change default callback to a wrapper that discards all but the
first arg.  This effectively declares that `erc-data' values for
nicknames may contain more than one element in the near future.
(erc-button--perform-nick-popup): New default nick-button callback
function wrapping `erc-nick-popup' in order to adapt it to the
`erc-button-nickname-callback-function' interface.  (Bug#60933)