Improve or remove code examples for TripleDES, DES, and RC2.

We have a number of examples for using these symmetric ciphers that I think either need to be significantly improved, or removed.

1. They demonstrate incorrect use of `CryptoStream`. Ex: https://github.com/dotnet/dotnet-api-docs/blob/13d97c7e536ce822c000883dba873a79d5d6e841/snippets/csharp/System.Security.Cryptography/DES/Create/memoryexample.cs#L99
2. They are incomplete examples. They, for example, do not demonstrate proper nonce management, they do not demonstrate proper authentication (authentication in cryptographic terms). It would be rather hard to demonstrate how to do these things properly in short examples, and nor do I think it's something that should be in API documentation.

The `CryptoStream` issue doesn't apply to `Aes` because we don't have that example for `Aes`, but we do have them for the other symmetric ciphers.

If we want to keep the examples, then they need to be re-written to 1. be correct 2. use modern APIs and 3. be heavily caveated that they are incomplete examples and do not demonstrate safe examples of using symmetric encryption.

/cc @bartonjs @GrabYourPitchforks