opt: turn on zigzag joins by default

[jseldess]

PR: cockroachdb/cockroach#35911

From release notes:

Zigzag joins will now be planned by the optimizer in certain cases. They were previously available via the experimental_enable_zigzag_join session variable and can still be disabled by setting that value to false. [#35911][#35911] {% comment %}doc{% endcomment %}