Support either `netstandard` or `net472` on `Microsoft.Extensions.ServiceDiscovery`

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Is your feature request related to a problem? Please describe the problem.

We have a ~190 project monorepo today containing a mix of `.NET9` and `.NET472` projects (and many `netstandard2.0` libraries). I was reading on the `Microsoft.Extensions.ServiceDiscovery` library and I could see that being quite useful for us even without the full Aspire integration.

However, I immediately noticed that the package is limited to .NET8+ today, which would make it unusable for us in cases where there is anything shared with our `.NET472` projects.

It would be very nice if the library supported either `netstandard2.0` or at least `net472` directly (which would allow us to at the very least switch to multi-targeting in a few cases).

### Describe the solution you'd like

We would like for `Microsoft.Extensions.ServiceDiscovery` to also support `netstandard2.0` or `net472` targets so that we could use it in more of our projects.

### Additional context

We rely _heavily_ on other `Microsoft.Extensions.*` packages even in our .NET Framework projects. We've migrated fully to using dependency injection, `IConfiguration`, `IOptions`, and even partially use `HostApplicationBuilder` for legacy projects, including many things built on top of those abstractions, such as `OpenTelemetry.Hosting.Extensions`.

From what I can see, `Microsoft.Extensions.ServiceDiscovery` only depends on those core abstractions which all already support `.netstandard` and `net461`+. Feels unusual to see this one _only_ targeting `.net8`+ based on that.

I understand this was implemented as part of Aspire, but it can _clearly_ be used independently from that framework.