Release ERC 5.6

Release ERC 5.6

* lisp/erc/erc.el: Change Version header from 5.6-git to 5.6.  A
`customize-package-emacs-version-alist' mapping for ERC 5.6 to Emacs
30.1 was already added when moving from 5.5 to 5.6-git.  The ERCVER
variable in doc/misc/erc.texi was likewise updated at that time.
Going forward, such changes will continue to happen preemptively, on
the flip side of each release.
(erc-version): Change version from 5.6-git to 5.6.

(cherry picked from commit d1e98a8f0128747030e97ffea20a8f67aa0cd923)