Move tls.el and starttls.el to lisp/obsolete/ (Bug#31457)

Move tls.el and starttls.el to lisp/obsolete/ (Bug#31457)

* lisp/obsolete/tls.el: Moved from lisp/net/tls.el.
* lisp/gnus/nnimap.el:
* lisp/url/url-http.el: Don't require tls, since it's obsolete.
* lisp/net/network-stream.el: Only require tls if we actually try to
use it (i.e., when (gnutls-available-p) returns nil).  Declare some
functions to fix compilation warnings.

* lisp/obsolete/starttls.el: Moved from lisp/net/starttls.el.
* lisp/net/sieve-manage.el:
* lisp/net/network-stream.el: Don't require `starttls' at the
top-level, declare the variables and functions used instead.
(network-stream-open-starttls): Only require `starttls' if
needed (i.e., gnutls-available-p fails).

* etc/NEWS: Announce obsoletion.