replace rijndael etc

[tdykstra]

Contributes to dotnet/docs#10199

From the issue:

Samples should not use any of the following cryptography types:

• RC2
• DES
• TripleDES
• Rijndael

As an alternative, they should reference Aes.

Notes:

• Samples for symmetricalgorithm.create and cryptography.keysizes create the default symmetric algorithm which is said to be Rijndael, but these don't even run -- they raise an exception saying the platform doesn't allow creation of default algorithm. So I didn't change these.
• The sample for symmetricalgorithm.legalkeysizes displays key sizes for the four deprecated symmetric algorithms as well as AES. I left it unchanged.
• The sample for passwordderivebytes uses TripleDes but when I try switching it to AES it fails with the error message "Object identifier (OID) is unknown". I left it unchanged.

[tdykstra]

This is ready for review. I'll make a separate PR for instances of Rijndael in XML encryption samples.