Add renovate config

[michalchecinski]

trafficstars

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 once a month, creating a single PR with everything available to update. This PR includes a minimal sensible config for me -- @michalchecinski. Later down the road, we can fine-tune Renovate by updating the config.

Code changes

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

Screenshots

Example PR on mobile fork:

Before you submit

• [ ] I have checked for formatting errors (dotnet tool run dotnet-format --check) (required)
• [ ] 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)