Support for OpenSSL 3.1

[jumaffre]

This also removes sha256_compress_openssl which isn't used by https://github.com/microsoft/ccf and SHA256_Transform doesn't have an equivalent EVP_ API in OpenSSL 3.1.

[jumaffre]

For the record, no performance impact (if anything, a small increase):

main:

2023-08-18T14:35:41.3308906Z 11: --- merklecpp trees with SHA256 compression function: 
2023-08-18T14:35:52.7836664Z 11: merklecpp : 16777216 insertions, 16385 roots in 11.4527 sec
2023-08-18T14:36:01.1910775Z 11: OpenSSL   : 16777216 insertions, 16385 roots in 7.06316 sec [REMOVED]
2023-08-18T14:36:14.1644787Z 11: mbedTLS   : 16777216 insertions, 16385 roots in 10.526 sec
2023-08-18T14:36:15.4520334Z 11: --- merklecpp trees with full SHA256: 
2023-08-18T14:36:38.4585162Z 11: OpenSSL   : 16777216 insertions, 16385 roots in 23.0065 sec <<< LOOK HERE
2023-08-18T14:36:57.7761980Z 11: mbedTLS   : 16777216 insertions, 16385 roots in 16.9856 sec
2023-08-18T14:37:00.3033531Z 11/11 Test #11: compare_hash_functions ...........   Passed   95.06 sec

This PR:

2023-08-18T14:36:56.3290685Z 11: Test command: /home/runner/work/merklecpp/merklecpp/build/Release/test/compare_hash_functions
2023-08-18T14:36:56.3293039Z 11: Working Directory: /home/runner/work/merklecpp/merklecpp/build/Release/test
2023-08-18T14:36:56.3293604Z 11: Test timeout computed to be: 10000000
2023-08-18T14:37:07.0230508Z 11: 4096 trees, 8390656 inserts, 67584 roots with SHA256 compression function: OK
2023-08-18T14:37:07.4985299Z 11: --- merklecpp trees with SHA256 compression function: 
2023-08-18T14:37:18.4600046Z 11: merklecpp : 16777216 insertions, 16385 roots in 10.9615 sec
2023-08-18T14:37:29.2683280Z 11: mbedTLS   : 16777216 insertions, 16385 roots in 9.61122 sec
2023-08-18T14:37:31.2392455Z 11: --- merklecpp trees with full SHA256: 
2023-08-18T14:37:51.5255251Z 11: OpenSSL   : 16777216 insertions, 16385 roots in 20.2863 sec <<< LOOK HERE
2023-08-18T14:38:09.8498516Z 11: mbedTLS   : 16777216 insertions, 16385 roots in 16.3014 sec
2023-08-18T14:38:12.0421059Z 11/11 Test #11: compare_hash_functions ...........   Passed   75.71 sec