Conditionalize the JIT setting

[bjester]

Summary

• We need to rollback the hotfixes database to v9.6, but the JIT change showed clear benefit, so making this conditional to avoid breaking things still on 9.6

References

#4834

Reviewer guidance

Local

I verified with no observed issues:

1. Started up devserver
2. Signed in
3. Created a new channel

Remote

Verified in python shell that this works:

>>> from django.db.backends.postgresql.features import DatabaseFeatures
>>> f = DatabaseFeatures(connection)
>>> f.is_postgresql_12
True
>>> f.is_postgresql_13
False
>>> f.is_postgresql_11
True

[rtibbles]

Didn't see it in the documentation, but this property is indeed in the source! https://github.com/django/django/blob/stable/3.2.x/django/db/backends/postgresql/features.py#L91

Looks good to me.