BreakingChanges: announce switch to "reftable" format

BreakingChanges: announce switch to "reftable" format

The "reftable" format has come a long way and has matured nicely since
it has been merged into git via 57db2a094d5 (refs: introduce reftable
backend, 2024-02-07). It fixes longstanding issues that cannot be fixed
with the "files" format in a backwards-compatible way and performs
significantly better in many use cases.

Announce that we will switch to the "reftable" format in Git 3.0 for
newly created repositories and wire up the change, hidden behind the
WITH_BREAKING_CHANGES preprocessor define.

This switch is dependent on support in the larger Git ecosystem. Most
importantly, libraries like JGit, libgit2 and Gitoxide should support
the reftable backend so that we don't break all applications and tools
built on top of those libraries.

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