Deprecate SpongyCastle

[LossyDragon]

Reference: #261

Spongy Castle is old and hasn't been updated since 2017. There could be a risk for security issues using it, though I am no expert with the Castles to know how much integration is there just for a Cipher instance at minimum.

This was a legacy support Android API running lower than Honeycomb. Google has since renamed their BC package since; even though that is now deprecated too.

Using normal BouncyCastle for Android is recommended based on many discussions while researching this subject. Android development has a support floor for API 21 (API 24 is starting to become the new floor too with dependencies) so this widens the gap from the original BC issue earlier versions of android had.

See: https://stackoverflow.com/a/66323575/13225929

Solution:
Keep SC as an option to use, but adjust CrytpoHelper to look for BC first if running android.
Update wiki and readme to reflect changes