Blake2b: Use OSSL_DIGEST_PARAM_SIZE as settable instead of XOFLEN

Blake2b: Use OSSL_DIGEST_PARAM_SIZE as settable instead of XOFLEN

BLAKE2 is not really an extensible output function unlike SHAKE
as the digest size must be set during the context initialization.
Thus it makes no sense to use OSSL_DIGEST_PARAM_XOFLEN.

We also need to adjust EVP_DigestFinal_ex() to query the
OSSL_DIGEST_PARAM_SIZE as gettable ctx param for the size.

Fixes #22488

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22491)