Update System.Reflection.Metadata dependency

[marek-safar]

Version Used: 2.6.1

Roslyn builds and ships using old System.Reflection.Metadata dependency. The fix was made to make System.Reflection.Metadata work on big-endian systems but Roslyn is still using very old version.

This blocks using Roslyn on big-endian systems and also makes features/enhancements like #24621 testing harder.

/cc @jaredpar

[tmat]

Once we update to the latest we can clean up: #24712

[agocke]

Also #23762

[tmat]

And dotnet/corefx#27589

[davidwrighton]

And #25185

[tmat]

And https://github.com/dotnet/corefx/issues/1815: http://source.roslyn.io/#Microsoft.CodeAnalysis/MetadataReference/ModuleMetadata.cs,170

[tmat]

Fixed by #27023