C# write benchmark improvements

[jtattermusch]

• target netcoreapp3.1 (the newest LTS of .NET core)
• add a few more useful write benchmarks
• use BinaryPrimitives.WriteUInt64LittleEndian for writing fixed64 values as starting from .NET Core 3.x, this is much faster than the current workaround.

Changes are mostly taken from JamesNK@337780c (as recommended here #7576 (comment) for more context). Thanks @JamesNK for the improvements!

[jtattermusch]

@JamesNK can you please review?

[jtattermusch]

Comparison for using BinaryPrimitives.WriteUInt64LittleEndian on .NET Core 3.1

BenchmarkDotNet=v0.12.1, OS=Windows 10.0.18362.900 (1903/May2019Update/19H1)
Intel Core i7-7600U CPU 2.80GHz (Kaby Lake), 1 CPU, 4 logical and 2 physical cores
.NET Core SDK=3.1.301
  [Host]     : .NET Core 3.1.5 (CoreCLR 4.700.20.26901, CoreFX 4.700.20.27001), X64 RyuJIT
  DefaultJob : .NET Core 3.1.5 (CoreCLR 4.700.20.26901, CoreFX 4.700.20.27001), X64 RyuJIT

BEFORE:

|                    Method | BytesToWrite |     Mean |    Error |   StdDev | Gen 0 | Gen 1 | Gen 2 | Allocated |
|-------------------------- |------------- |---------:|---------:|---------:|------:|------:|------:|----------:|
| WriteFixed64_WriteContext |        10080 | 14.97 us | 0.156 us | 0.138 us |     - |     - |     - |         - |

AFTER:

|                    Method | BytesToWrite |     Mean |     Error |    StdDev | Gen 0 | Gen 1 | Gen 2 | Allocated |
|-------------------------- |------------- |---------:|----------:|----------:|------:|------:|------:|----------:|
| WriteFixed64_WriteContext |        10080 | 3.097 us | 0.0603 us | 0.1071 us |     - |     - |     - |         - |

[jtattermusch]

the CI doesn't have a new-enough version of the SDK (which is something that would need to be fixed first).

dotnet restore csharp/src/Google.Protobuf.sln
/root/.dotnet/sdk/3.0.100/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.TargetFrameworkInference.targets(127,5): error NETSDK1045: The current .NET SDK does not support targeting .NET Core 3.1.  Either target .NET Core 3.0 or lower, or use a version of the .NET SDK that supports .NET Core 3.1. [/tmp/protobuf/protobuf/csharp/src/Google.Protobuf.Benchmarks/Google.Protobuf.Benchmarks.csproj]

[JamesNK]

Opps, I saw 3.x in global.json and assumed it was 3.1.

Build should update to 3.1. It is a LTS release. 3.0 is no longer supported.