Cache legacy keys instead of downgrading them

Cache legacy keys instead of downgrading them

If someone calls an EVP_PKEY_get0*() function then we create a legacy
key and cache it in the EVP_PKEY - but it doesn't become an "origin" and
it doesn't ever get updated. This will be documented as a restriction of
the EVP_PKEY_get0*() function with provided keys.

Fixes #14020

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/14319)