[IMPROVEMENT] Migrate nuget dependency management from Dependabot to Renovate

[snapsl]

Please confirm these before moving forward.

• I have searched for my feature proposal and have not found a work-in-progress/duplicate/resolved/discarded issue.
• This improvement refers to an existing feature. If you want to suggest a new feature, please use this template.
• This improvement is not a bug. If you want to report a bug, please use this template.

Describe the improvement

As Dependabot is currently failing to detect NuGet dependency updates, Renovate could be an effective alternative.
See https://github.com/snapsl/UniGetUI/pulls.

Describe how this improvement could help users

.