## Problem
part of https://github.com/neondatabase/neon/issues/9516
One thing I realized in the past few months is that "no-way-back" things
like this are scary to roll out without a fine-grained rollout infra.
The plan was to flip the flag in the repo and roll it out soon, but I
don't think rolling out would happen in the near future. So I'd rather
revert the flag to avoid creating a discrepancy between staging and the
regress tests.
## Summary of changes
Not using rel_size_v2 by default in unit tests; we still have a few
tests to explicitly test the new format so we still get some test
coverages.
---------
Signed-off-by: Alex Chi Z <chi@neon.tech>