Make use of Django LTS only preset

As #140 notes, we're getting renovate offering to upgrade to Django 6.0 which it shouldn't do.

We have an allowOnlyLTSDjango preset in our DevOps renovate config which is intended for use in this case to only propose updating to Django LTS releases.

Add this preset both to this project's config (to avoid updating Django in the boilerplate) and the boilerplate template's config (to avoid updating in downstream repos).