Cleanup KurrentDB integration

[larsfjerm]

Closes #929

• Use KurrentDB vars instead of outdated EventStore vars from Add KurrentDB integration and mark EventStore as obsolete #895.
• Updated client to latest verison.
• Replaced external health check dependency to use the KurrentDB.Client dependency only.

PR Checklist

• Created a feature/dev branch in your fork (vs. submitting directly from a commit on main)
• Based off latest main branch of toolkit
• PR doesn't include merge commits (always rebase on top of our main, if needed)
• New integration
  • Docs are written
  • Added description of major feature to project description for NuGet package (4000 total character limit, so don't push entire description over that)
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Contains NO breaking changes
• Every new API (including internal ones) has full XML docs
• Code follows all style conventions

Other information