refs/files: deprecate writing symrefs as symbolic links

refs/files: deprecate writing symrefs as symbolic links

The "files" backend has the ability to store symbolic refs as symbolic
links, which can be configured via "core.preferSymlinkRefs". This
feature stems back from the early days: the initial implementation of
symbolic refs used symlinks exclusively. The symref format was only
introduced in 9b143c6e15 (Teach update-ref about a symbolic ref stored
in a textfile., 2005-09-25) and made the default in 9f0bb90d16
(core.prefersymlinkrefs: use symlinks for .git/HEAD, 2006-05-02).

This is all about 20 years ago, and there are no known reasons nowadays
why one would want to use symlinks instead of symrefs. Mark the feature
for deprecation in Git 3.0.

Note that this only deprecates _writing_ symrefs as symbolic links.
Reading such symrefs is still supported for now.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>