chore: Conditional build targets for linux (#204)

When on linux this doesn't try to load the windows or mac targets. This
way shenanigans aren't needed to just load a solution on linux and work.
On the mac everything will load.

We could add something similar for windows, but currently nobody is
trying to work on the client SDK in windows.

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> On Linux, only build `netstandard2.0` and `net8.0` by adding a
Linux-specific `BuildFrameworks` condition in the project file.
> 
> - **Build configuration**:
> - Add Linux-specific `BuildFrameworks` in
`pkgs/sdk/client/src/LaunchDarkly.ClientSdk.csproj` to build only
`netstandard2.0;net8.0`, avoiding Android/iOS/Mac/Windows targets on
Linux.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
7824e8a. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: kinyoklion

pkgs/sdk/client/src/LaunchDarkly.ClientSdk.csproj

@@ -10,6 +10,7 @@
              that doesn't have the MAUI tools installed. That is currently the case in
              the release phase where we build HTML documentation. -->
 
+        <BuildFrameworks Condition="'$(BUILDFRAMEWORKS)' == '' and $([System.Runtime.InteropServices.RuntimeInformation]::IsOSPlatform($([System.Runtime.InteropServices.OSPlatform]::Linux)))">netstandard2.0;net8.0</BuildFrameworks>
         <BuildFrameworks Condition="'$(BUILDFRAMEWORKS)' == ''">netstandard2.0;net8.0;net8.0-android;net8.0-ios;net8.0-maccatalyst;net8.0-windows</BuildFrameworks>
         <TargetFrameworks>$(BUILDFRAMEWORKS)</TargetFrameworks>
         <OutputType>Library</OutputType>