Fix sample output for .NET Core on SymmetricAlgorithm.BlockSize

[vcsjones]

Summary

The output of the sample program for BlockSize will vary depending on if you run it for .NET Framework or .NET Core for RijndaelManaged (the max block size is 128 in .NET Core, while it is 256 in .NET Framework). This makes note of that difference.

Fixes dotnet/runtime#37127

/cc @bartonjs

[mairaw]

This example has a VB counterpart, which we try to keep them in sync:
https://github.com/dotnet/dotnet-api-docs/blob/master/samples/snippets/visualbasic/VS_Snippets_CLR_System/system.security.cryptography.symmetricalgorithm.blocksize/vb/program.vb

[vcsjones]

@mairaw I may be misunderstanding but the PR has changes for both C# and VB?

https://github.com/dotnet/dotnet-api-docs/pull/4300/files#diff-0ce01c0614677250518b7c1dcb4e7032

[BillWagner]

@vcsjones The snippet file changed in this PR has a corresponding VB file. Those are inputs to the docs build, and the code is rendered in the final page. You should make the corresponding change in the VB file so that both examples contain the update when rendered.

[mairaw]

My bad @vcsjones! Somehow I missed that.