Documentation/BreakingChanges: announce removal of git-pack-redundant(1)

Documentation/BreakingChanges: announce removal of git-pack-redundant(1)

The git-pack-redundant(1) command is already in the process of being
phased out and dies unless the user passes the `--i-still-use-this` flag
since 4406522b76 (pack-redundant: escalate deprecation warning to an
error, 2023-03-23). We haven't heard any complaints, so let's announce
the removal of this command in Git 3.0 in our breaking changes document.

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