feat: Update target frameworks to net8.0

[jsonbailey]

Overview

Updates dotnet-sdk-internal to support net8.0, matching the target framework set used by the dotnet-core packages (server-ai, telemetry, server, common).

Resolves SDK-1435.

Target frameworks: netstandard2.0;netcoreapp3.1;net462;net6.0 → netstandard2.0;net462;net8.0

Not a breaking change

Dropping netcoreapp3.1 and net6.0 does not warrant a major version bump:

• netstandard2.0 and net462 continue to cover .NET Framework consumers.
• netcoreapp3.1 is not used by the consuming SDKs.
• Both dropped frameworks are out of support, and removing out-of-support TFMs is permitted without a major version.

The CS8352/CS8350 problem (why this stalled previously)

The JsonConverterHelpers ref-struct helpers (ArrayHelper/ObjectHelper) could not be consumed under net8.0: the .NET 7+ runtime enables byref-field ref-safety analysis regardless of LangVersion, so the compiler assumed these ref structs might capture a reference to the Utf8JsonReader passed to their methods, producing CS8350/CS8352 at every call site (including plain inline Next(ref reader) calls — so every downstream consumer would hit it on net8.0).

Fix: annotate the Utf8JsonReader parameters as scoped to declare that the helpers never capture a reference to the reader (which is true), and raise LangVersion to 11 so the annotation is available. scoped is a compile-time-only annotation — verified to build cleanly on netstandard2.0 and net462 as well. There is direct precedent: dotnet-core's server-ai ships LangVersion 11 with the identical TFM set.

Other changes

• HttpProperties: simplified the handler guard #if NETCOREAPP || NET6_0 → #if NETCOREAPP (the branch selects SocketsHttpHandler, available on all .NET Core 2.1+/.NET 5+ targets, so no version floor applies; the || NET6_0 clause was redundant).
• CI and release actions updated to install the 8.0 SDK and target net8.0.

Testing

• net8.0: 180/180 unit tests pass.
• netstandard2.0 and net462: build clean.
• No tests were removed or disabled.

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Note

Medium Risk
TFM matrix changes can affect consumers that targeted net6.0/netcoreapp3.1 directly; public helper signatures changed to scoped ref, which downstream code must compile against on modern runtimes.

Overview
Aligns LaunchDarkly.InternalSdk with the dotnet-core TFM set by moving the modern target from net6.0 / netcoreapp3.1 to net8.0 while keeping netstandard2.0 and net462. Default build/test frameworks and GitHub Actions (setup-dotnet 8.0, CI matrix, full release framework_target) are updated accordingly; LangVersion is raised to 11.

JsonConverterHelpers ArrayHelper / ObjectHelper APIs now use scoped ref Utf8JsonReader so net8.0 ref-safety analysis (CS8350 / CS8352) passes without changing runtime behavior. Tests mirror those signatures.

HttpProperties uses #if NETCOREAPP only (drops redundant NET6_0). System.Collections.Immutable is no longer pulled in for the removed netcoreapp3.1 TFM.

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