Move FFDH to decrepit

[reaperhulk]

Addresses #15603.

Diffie-Hellman over finite fields (FFDH) was deprecated in 50.0.0 on the way to removal, but some protocols mandate it with no alternative to negotiate into. This moves it into cryptography.hazmat.decrepit.asymmetric.dh, following the pattern used for the decrepit ciphers and modes, and keeps every existing deprecation warning with reworded text: FFDH is deprecated and will only be available from the decrepit module starting in 53.0.0.

What moved

• cryptography.hazmat.decrepit.asymmetric.dh now holds generate_parameters, the DHParameters / DHPrivateKey / DHPublicKey ABCs, the three numbers classes, and load_pem_parameters / load_der_parameters (these can only load FFDH parameters, so they belong with it). Nothing in the decrepit module warns.
• cryptography.hazmat.primitives.asymmetric.dh becomes a shim that re-exports the same objects via utils.deprecated, so isinstance checks against either path keep working. The warning class is DeprecatedIn51 with removal targeted at 53.0.0, matching the two-release window.
• serialization.load_pem_parameters / load_der_parameters stay deprecated, with the "has been moved to ... and will be removed from ... in 53.0.0" wording the Camellia and CFB/OFB moves used.
• The Rust warning emitted when the generic key loading APIs (and X.509 public_key()) hand back a DH key is kept and reworded the same way. DEPRECATED_IN_50 in types.rs was only used there and is removed.
• asymmetric.types now takes the DH types from the decrepit module instead of the _DHPublicKey / _DHPrivateKey private aliases, which are dropped.

Bug fixed along the way

The old shim (and dsa.py today, not touched here) called utils.deprecated(DHParameters, ..., name="DHParametersWithSerialization") after utils.deprecated(DHParameters, ..., name="DHParameters") had already replaced the module global with the _DeprecatedValue marker, so the ...WithSerialization aliases resolved to the marker object instead of the class. The new shim reads every value from the decrepit module directly, and the new test covers all ten names.

Docs and tests

• New docs/hazmat/decrepit/dh.rst documents the decrepit module (with versionadded:: 51.0.0 markers and doctests for the parameter loaders). The old primitives/asymmetric/dh.rst keeps its content with a "moved to decrepit" warning, as the cipher and mode pages do, so older changelog cross-references still resolve under nitpicky mode.
• Tests import from the decrepit module and no longer need to suppress warnings when building fixtures. TestFFDHDeprecation checks the old module names, the serialization loaders, the generic key loaders, and that the decrepit path is warning-free.

One design point for reviewers

Loading FFDH keys through load_pem_private_key and friends still warns, as requested ("keep the warnings"). The decrepit module deliberately does not grow its own PEM/DER key loaders in this PR, so if the intent for 53.0.0 is that the generic loaders stop returning DH keys, a follow-up would need to add those; if instead the generic loaders should keep working and return the decrepit types, the Rust warning can simply be removed later.

nox -e local passes in full. nox -e docs builds cleanly under -W; the only doctest failures locally are in unrelated pages (Argon2, AES-GCM-SIV, SHAKE squeeze) because the container's system OpenSSL 3.0.13 lacks them.

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https://claude.ai/code/session_01TmnZQpY2o4HeBNFN5tfqYE

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[alex]

Ugh, I really feel like this is moving backwards on the deprecation, and therefore suggests we're prepared to carry it forever, which I really don't want to be.