Disable OpenSSL EVP digest padding in pgcrypto

Disable OpenSSL EVP digest padding in pgcrypto

The PX layer in pgcrypto is handling digest padding on its own uniformly
for all backend implementations. Starting with OpenSSL 3.0.0, DecryptUpdate
doesn't flush the last block in case padding is enabled so explicitly
disable it as we don't use it.

This will be backpatched to all supported version once there is sufficient
testing in the buildfarm of OpenSSL 3.

Reviewed-by: Peter Eisentraut, Michael Paquier
Discussion: https://postgr.es/m/FEF81714-D479-4512-839B-C769D2605F8A@yesql.se
Backpatch-through: 9.6