feat: prevent upgrades to non-LTS Django releases

As noted in #140, we're getting renovate MRs asking to upgrade to Django 6.0 which is a non-LTS release. We have an allowOnlyLTSDjango preset available intended for use in these cases. Make use of it both in the generated template (to avoid downstream MR creation) and in this project (to avoid bumping the version in new templates).

Closes #148 (closed)