Add multi-targeting for .NET 8.0 and 6.0 LTS

[jmarbutt]

Problem

The package currently only targets net9.0 (preview), which significantly limits adoption and production readiness.

Location: Inngest/Inngest.csproj:4

<TargetFramework>net9.0</TargetFramework>

Suggested Solution

Multi-target to include current LTS versions:

<TargetFrameworks>net9.0;net8.0;net6.0</TargetFrameworks>

This may require:

• Conditional package references for version-specific dependencies
• Testing on all target frameworks
• Potentially using polyfills for newer APIs

Priority

High - most production .NET applications run on LTS versions (8.0 or 6.0).