Deprecate erc-server-alist and erc-server-select

Deprecate erc-server-alist and erc-server-select

* etc/ERC-NEWS: Announce deprecation of `erc-server-alist' and
`erc-server-select'.
* lisp/erc/erc-networks.el: Comment out call to `erc-get' at end of
file.
(erc-server-alist) Change shape to accommodate a fifth member: TLS
ports.  Add default TLS ports for Libera.Chat and OFTC.  Deprecate
option.
(erc-ports-list): Overload for internal use to accept a number instead
of a list, but don't advertise this fact.
(erc-networks--server-select): Convert `erc-server-select' into a
function that performs the same prompting but returns a full URL or a
host name instead of calling `erc'.
(erc-server-select): Move to erc.el.
* lisp/erc/erc.el (erc--prompt-for-server-functions): New variable to
allow callers of `erc-select-read-args' to affect how server-prompting
is handled without adding additional params.
(erc-select-read-args): Defer to `erc--prompt-for-server-function'
when non-nil.
(erc-server-select): New transplanted function, a deprecated, now
TSL-aware version of the old quirky entry point from erc-networks.el.
Reimplemented as a simple wrapper for `erc'.
* test/lisp/erc/erc-networks-tests.el (erc-ports-list): New test.
* test/lisp/erc/erc-tests.el (erc-server-select): New test.
(Bug#64478)