Merge from origin/emacs-28

ac06608878 (origin/emacs-28) Release ERC 5.4
a1a589d07b * etc/ERC-NEWS: Announce ERC's addition to GNU ELPA.
6c7947f0a1 * etc/ERC-NEWS: Fix outline level for the recent additions.
c480b68644 Add ERC entries for 'customize-package-emacs-version-alist'
4afff515c8 Expand the full file name
36a485a1af Obsolete XEmacs compat convention in 'erc-button-press-but...