feat: Add .NET OpenFeature server provider examples

[jsonbailey]

Summary

Migrates the example from launchdarkly/hello-openfeature-dotnet-server into examples-dotnet, following the EXAM layout.

Layout

openfeature-server-provider/
├── README.md                    # index (Getting Started)
└── getting-started/             # Flag Retrieval
    ├── HelloOpenFeatureDotnet.csproj
    ├── Hello.cs
    ├── README.md
    └── NOTICE                   # Copyright 2022 (from the old repo)

• Partition named after the package (LaunchDarkly.OpenFeature.ServerProvider → openfeature-server-provider/).
• Modernized to net8.0 only — the original multi-targeted net471;net6.0;net8.0; trimmed to net8.0 to align with the other examples (we don't publish these, so no broad TFM coverage needed).
• Root README gains the OpenFeature row; per-example NOTICE carries the old repo's 2022 copyright; BOM stripped.
• Verified: dotnet build passes.

Notes

• Build CI: this example is auto-covered by build.yml's glob once feat: Add .NET Server AI SDK examples #2 (which introduces build.yml) merges. Verified the build locally in the meantime.
• The old repo ran the example via verify-hello-app (server key); extending verify-hello.yml to run it too can be a follow-up.

Part of SDK-2559. Closes the migration portion of SDK-2562 (old-repo README repoint + Terraform archive tracked separately on that ticket).

Jira: SDK-2562

[jsonbailey]

Updated to per-slice CI: this PR now includes its own path-filtered openfeature-server-provider.yml that builds only the openfeature-server-provider/ examples. This supersedes the earlier note about build CI arriving via #2's build.yml.