MIDX: revert the default version to v1

We introduced midx version 2 in b2ec8e90c2 (midx: do not require packs
to be sorted in lexicographic order, 2026-02-24) and now write it by
default. The rationale was that older versions should ignore the v2 midx
and fall back to using the packs (just like we do for other midx
errors). Unfortunately this is not the case, as we have a hard die()
when we see an unknown midx version.

As a result, writing a midx with Git 2.54-rc2 puts the repository into a
state that is unusable with Git 2.53. And this midx write may happen
behind the scenes as part of normal operations, like fetch.

Let's switch back to writing v1 by default to avoid regressing the case
where multiple versions of Git are used on the same repository.

There is one gotcha, though: the v2 format is required for some new
features, like midx compaction, and running "git multi-pack-index
compact" will complain when asked to write a v1 index. The user must set
midx.version to "2" to make the feature work.

So instead of always using v1, we'll base the default on whether the
requested feature requires v2. That does mean that running midx
compaction will create a repository that can't be read by older versions
of Git. But we never do that by default; only people experimenting with
the new feature will be affected.

We have to adjust the test expectation in t5319, since it will now
generate v1 files. And our "auto-select v2" is covered by the tests in
t5335, which continue to check that compaction works without having to
set midx.version manually (and also explicitly check that asking for v1
with compaction reports the problem).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>