Mark RSA-2048 as legacy

[martgil]

Hi @martgil, by default FlowCrypt offers to generate curve25519 key, it should work well for most users.
As I remember, we left rsa2048 for compatibility reasons, as some users have company policies requiring such keys.
So while it's not recommended to use this algorithm - it's still used by some users.
As a small improvement, we can move it to the bottom of algorithm options list and label it as RSA2048 (deprecated)

Originally posted by @sosnovsky in #314

[martgil]

Hello @sosnovsky, I just realized that it would be more appropriate to label it as “legacy” instead of “deprecated,” since RSA-2048 is not deprecated as of today by current standards.