Add config to skip non LTS django versions

Broken out from https://gitlab.developers.cam.ac.uk/uis/devops/iam/authentication/shibboleth/shibboleth-metadata/-/merge_requests/53#note_712446:

I suspect we have to maintain the list manually because Python's packaging doesn't really have the concept of an LTS which can be understood by machines. We probably want to fail safe and so encode that one should skip non-LTS versions (according to Django's docs). Then if we forget to update the list our failure mode is to upgrade, rather than to stop upgrades happening.

How about a allowOnlyDjangoLTS preset in the renovate-config repo with something like:

{
  "packageRules": [
    {
      "matchPackageNames": ["django"],
      "allowedVersions": "!/([56]\\.[01]\\.*)|(7\\.0\\.*)/"
    }
  ]
}

That should catch 5.2 and 6.2 which are LTS releases but skip 5.0, 5.1, 6.0, 6.1 and 7.0.