Spring Security Update Encryptors documentation for standard and stronger

Summary

This involves updating the reference documentation and the Javadocs for Encryptors.stronger and Encryptor.standard.

Currently, the reference documentation only mentions the standard(CBC) encryptor. We should mention the stronger(GCM) encryptor and suggest that users favour this, as it is more secure.

We should also update the Javadocs for both standard and stronger in order to guide users to stronger if they are not already using it.

This issue was originally reported by Neil Madden, ForgeRock.