Feat: Add support for Kurrent

[rasmus]

Moving the old EventStore code to https://www.kurrent.io/

[rasmus]

.review

[github-actions]

Findings

• Source/EventFlow.Kurrent.Tests/IntegrationTests/KurrentEventStoreTests.cs:39 (High) – The override of Options(...) no longer calls the base implementation from TestSuiteForEventStore. That base method injects the ISubscribeSynchronousToAll mock which the suite relies on for assertions about published domain events. Without it, every test that inspects PublishedDomainEvents will fail. Call base.Options(eventFlowOptions) (and then append the Kurrent-specific wiring) to keep the subscriber registered.
• Source/EventFlow.Kurrent/EventFlow.Kurrent.csproj:3 (High) – The replacement for EventFlow.EventStores.EventStore now targets only net8.0, whereas the old package shipped as netstandard2.0. This drops support for the existing EventFlow hosting story on netcoreapp3.1/net6.0 and earlier, so anybody not yet on net8 can no longer use an EventStore-compatible backend. Please multi-target (or document/communicate a deliberate breaking change and bump accordingly) so the new package remains usable for the same consumer set.