Migrate to Renovate from Dependabot

Dependabot is the sole reason blocking us from using uv (see https://github.com/dependabot/dependabot-core/issues/10478).

Let's see if Renovate's up to the task.

Next steps:
- [ ] Reconfigure renovate in flagsmith-common to mimic the behaviour of dependabot (to reduce noise)
- [ ] Work through remainin python repositories to replace depenabot
- [ ] (Optional) work through other repositories to replace dependabot too?

Python repositories that need migrating:
- [ ] Flagsmith/edge-api
- [ ] Flagsmith/edge-proxy
- [ ] Flagsmith/flagsmith
- [ ] Flagsmith/flagsmith-admin-client
- [ ] Flagsmith/flagsmith-auth-controller
- [x] Flagsmith/flagsmith-common
- [ ] Flagsmith/flagsmith-ee
- [ ] Flagsmith/flagsmith-engine
- [ ] Flagsmith/flagsmith-ldap
- [ ] Flagsmith/flagsmith-python-client
- [ ] Flagsmith/flagsmith-rbac
- [ ] Flagsmith/flagsmith-saml
- [ ] Flagsmith/flagsmith-split-testing
- [ ] Flagsmith/flagsmith-sse
- [ ] Flagsmith/flagsmith-workflows