Change SHA2 implementation based on OpenSSL to use EVP digest routines

Change SHA2 implementation based on OpenSSL to use EVP digest routines

The use of low-level hash routines is not recommended by upstream
OpenSSL since 2000, and pgcrypto already switched to EVP as of 5ff4a67.
Note that this also fixes a failure with SCRAM authentication when using
FIPS in OpenSSL, but as there have been few complaints about this
problem and as this causes an ABI breakage, no backpatch is done.

Author: Michael Paquier, Alessandro Gherardi
Reviewed-by: Daniel Gustafsson
Discussion: https://postgr.es/m/20200924025314.GE7405@paquier.xyz
Discussion: https://postgr.es/m/20180911030250.GA27115@paquier.xyz