Add renovate config

[michalchecinski]

Type of change

• [ ] Bug fix
• [ ] New feature development
• [ ] Tech debt (refactoring, code cleanup, dependency upgrades, etc)
• [ ] Build/deploy pipeline (DevOps)
• [x] Other

Objective

Configure a dependency update using Renovate to run 1 time per month, creating a single PR with everything available to be updated. This PR includes a minimal sensible config for me -- @michalchecinski. Later down the road, we can fine-tune Renovate by updating the config.

Example PR created on server fork by Renovate:

Code changes

• renovate.json: Add config for Renovate to create a single PR once a month to update NuGet packages.

Before you submit

- [ ] I have checked for formatting errors (`dotnet format --verify-no-changes`) (required)
- [ ] If making database changes - I have also updated Entity Framework queries and/or migrations
- [ ] I have added **unit tests** where it makes sense to do so (encouraged but not required)
- [ ] This change requires a **documentation update** (notify the documentation team)
- [ ] This change has particular **deployment requirements** (notify the DevOps team)

[mimartin12]

LGTM, I will let @bitwarden/dept-development sign off on the approval. 👍