feat: Replace Dependabot with Renovate for GitHub Actions updates

[jsonbailey]

Summary

• Adds .github/renovate.json extending the new sdk-dotnet shared preset from gh-actions
• Removes .github/dependabot.yml — Renovate will manage GitHub Actions updates going forward
• nuget packages are disabled in the preset, so only GitHub Actions refs will get update PRs

Note: depends on launchdarkly/gh-actions#106 (sdk-dotnet preset) being merged first.

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Note

Low Risk
CI-only configuration swap with no application or runtime code changes; main caveat is depending on the external sdk-dotnet Renovate preset being available.

Overview
Replaces Dependabot with Renovate for automated update PRs in this repo.

Deletes .github/dependabot.yml, which only configured daily NuGet updates for getting-started/HelloDotNet. Adds .github/renovate.json that extends the shared github>launchdarkly/gh-actions//renovate/sdk-dotnet preset (requires that preset to land in gh-actions first). Per the PR description, that preset turns off NuGet Renovate updates, so ongoing automation here shifts to GitHub Actions ref bumps rather than HelloDotNet package updates.

^{Reviewed by Cursor Bugbot for commit eb7a9cd. Bugbot is set up for automated code reviews on this repo. Configure here.}

[jsonbailey]

Don't merge until the dependent PR is merged (launchdarkly/gh-actions#106)