Drop support for .NET 6 Mobile

[bruno-garcia]

Microsoft's support policy never affected Sentry's directly, We still support deprecated targets such as, .NET Core 3.1, .NET 5, etc. Because we have customers that use those. And because up until this point we were able to use the latest .NET SDK, in combination with deprecated runtimes, and build and test our library. Unfortunately with .NET 8, it won't be possible to do that with respect to mobile, and we'll need to drop .NET 6 Android, iOS, Mac Catalyst, etc.

The MAUI support policy is different than the standard .NET.

While .NET 6 is LTS and supported until November 12, 2024
.NET 6 MAUI is already out of support, since May 8, 2023

More details:

• .NET 8 RC1 silently breaks binding library (build successful but no binding generated) dotnet/android#8340
• [build] remove .NET 6 support dotnet/android#7900

[bruno-garcia]

Going live on 4.0.0 (see #2631)