Microsoft.Data.Sqlite: Multi-target .NET Standard 2.1

[bricelam]

This only matters if you're using SqliteBlob polymorphically as a Stream on older platforms. Everything is functional, you just get the perf/memory characteristics of the API before PR #24503. To illustrate, here's a stack trace:

   at Microsoft.Data.Sqlite.SqliteBlob.Write(ReadOnlySpan`1 buffer)
   at Microsoft.Data.Sqlite.SqliteBlob.Write(Byte[] buffer, Int32 offset, Int32 count)
   at System.IO.Stream.Write(ReadOnlySpan`1 buffer)

Platforms where this will matter:

• .NET Core 2.1-3.1
• Mono ~6.4+
• Xamarin.iOS ~12.16+ (< .NET 6)
• Xamarin.Mac ~5.16+ (< .NET 6)
• Xamarin.Android ~10.0+ (< .NET 6)

Multi-targeting .NET Standard 2.1 mitigates most of these, but (IMHO) this probably isn't worth it.

[bricelam]

We discussed this as a team and decided that there wasn't enough benefit to warrant adding another target. The workaround is just to cast to SqliteBlob if you really need to reduce memory allocations on these platforms.