Remove enable_dilithium flag

[jakemas]

Need to merge #2072 first

Issues:

It's time. (also we need to do this to add ML-DSA to the FIPS module)

Description of changes:

• Removed the enable_dilithium flag
• Added a description to APIs to indicate the external APIs may still be subject to change as the standards space develops. (We don't envision them changing, but they are extremely new).

Call-outs:

Removing the flag has little consequence -- other than it makes the APIs we expose in include/openssl/evp.h that much more "final". We should consider how much we like them before we commit to them. We made a point to refer to asymmetric keypairs as public and private keys, rather than public and secret keys. However, we haven't always been consistant with this, so there is a mix of both in the library. Users will find the consistency between EVP_PKEY_pqdsa_new_raw_secret_key and EVP_PKEY_kem_new_raw_secret_key more satisfying.

However, after internal discussions, we'd also like to change the name of EVP_PKEY_kem_new_raw_public_key and EVP_PKEY_kem_new_raw_secret_key to EVP_PKEY_kem_new_raw_encapsulation_key and EVP_PKEY_kem_new_raw_decapsulation_key to match the names within FIPS 203. Thus, we should stick with secret.

We did consider the use of an experimental flag (by using the OPENSSL_DEPRECATED alias), but, this seems a little over the top. If there are any discrepancy with the API down the line, we will make a change as we are proposing with the kem APIs above. Alternatively, we could place the EVP APIs in include/openssl/experimental/.

Testing:

To celebrate the removal of this flag, enjoy this haiku:

feature flag removed,
post-quantum strength stands alone,
simpler code prevails.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.93%. Comparing base (2329adb) to head (c1ad06d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2081      +/-   ##
==========================================
+ Coverage   78.75%   78.93%   +0.18%     
==========================================
  Files         598      610      +12     
  Lines      103650   105175    +1525     
  Branches    14719    14907     +188     
==========================================
+ Hits        81625    83023    +1398     
- Misses      21371    21500     +129     
+ Partials      654      652       -2     

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