PGP: Remove support for legacy X448, legacy Ed448?

[vanitasvitae]

I'm currently adding more tests, including for Legacy X448 encryption and I wonder, if it's really worth supporting non-standard legacy X488 (with PublicKeyAlgorithmTags.ECDH) and legacy Ed448 (with PublicKeyAlgorithmTags.EDDSA_LEGACY).

Originally I thought support for these would be desirable, since the PGPKeyConverter classes had partial support already, but the more I think about it I guess supporting those key types would lead to people generating them, causing interop issues with other implementations.
What do you think?

Removing support for non-standard keys would involve reverting some changes from #1663 (simplifying EDDSA_LEGACY branches to always assume Ed25519, removing X448 from the ECDH branches), and removing the LegacyX448KeyPairTest, LegacyEd448KeyPairTest classes.

Originally posted by @vanitasvitae in #1675 (comment)

[vanitasvitae]

I just noticed, that there appear to be legacy Ed448 keys already used in the codebase, e.g. here.
That makes the discussion more interesting :D

[vanitasvitae]

Turns out, LibrePGP defines use of Ed448 with LEGACY_EDDSA and X448 with ECDH (see here), so removing support is not really an option if we want to stay compatible to LibrePGP.