Add breaking change documentation for CoseSigner.Key property nullability

[Copilot]

This PR adds documentation for a breaking change in .NET 10 Preview 7 where the CoseSigner.Key property may now return null.

Changes

The System.Security.Cryptography.Cose.CoseSigner.Key property has changed from type AsymmetricAlgorithm to AsymmetricAlgorithm? to support Post-Quantum Cryptography (PQC) algorithms like ML-DSA that don't derive from AsymmetricAlgorithm.

Previous behavior:

using RSA rsaKey = RSA.Create();
CoseSigner signer = new CoseSigner(rsaKey, RSASignaturePadding.Pss, HashAlgorithmName.SHA512);
AsymmetricAlgorithm key = signer.Key; // key was never null

New behavior:

using MLDsa mldsa = MLDsa.GenerateKey(MLDsaAlgorithm.MLDsa44);
CoseKey coseKey = new CoseKey(mldsa);
CoseSigner signer = new CoseSigner(coseKey);
AsymmetricAlgorithm? key = signer.Key; // key is null for non-AsymmetricAlgorithm keys

Documentation updates

• Created: docs/core/compatibility/cryptography/10.0/cosesigner-key-null.md - Complete breaking change documentation following the standard template
• Updated: docs/core/compatibility/10.0.md - Added entry to the Cryptography section table
• Updated: docs/core/compatibility/toc.yml - Added navigation entry under Cryptography > .NET 10

The documentation explains the reason for the change (support for PQC algorithms), provides clear examples of both old and new behavior, and includes recommended actions for handling null values.

Fixes #46999.

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