FEATURE_FUNCPTRS/plain breaks when targeting net6.0+ on Windows

[filipnavara]

FEATURE_FUNCPTRS/plain was introduced in commit 9adcbfc to workaround a bug in iOS AOT compiler. That bug was fixed in .NET 7 and .NET 8 and so the workaround is no longer necessary. We should use FEATURE_FUNCPTRS/callingconv again for generic .NET 6+ platforms.

The issue is that compiling a unit test assembly against generic net6+ target and referencing SQLitePCLRaw.provider.e_sqlite3 on Windows results in using the generic net6.0 version from the NuGet package. That assembly version is missing the CDecl calling convention decorations on UnmanagedCallersOnly attributes and thus results in incorrect calling convention used on x86 platforms. This can be observed when using sqlite3_create_function to register custom functions since the callbacks corrupt the stack and result in access violation.

[ericsink]

This was included in 2.1.7, which has been pushed up to nuget.