Remove DSA from macOS

[vcsjones]

This removes DSA (FF-DSA) from macOS.

Closes #71738
Fixes #118107

[Copilot]

Pull Request Overview

This PR removes Digital Signature Algorithm (DSA) support from macOS platforms in the .NET cryptography libraries. The change affects the native Apple cryptography implementation and updates test files to properly handle the absence of DSA support.

Key changes include:

• Removal of DSA-specific enum values and code from the native Apple cryptography implementation
• Updates to test attributes to use conditional testing based on platform DSA support
• Deletion of DSA SecurityTransforms implementation files
• Updates to platform support detection to exclude Apple platforms from DSA support

Reviewed Changes

Copilot reviewed 49 out of 49 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/native/libs/System.Security.Cryptography.Native.Apple/pal_signverify.h	Removes DSA enum value and updates function documentation
src/native/libs/System.Security.Cryptography.Native.Apple/pal_signverify.c	Removes DSA-specific transform functions and conditional logic
src/native/libs/System.Security.Cryptography.Native.Apple/pal_seckey.h	Updates documentation to remove DSA references
src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/X509Certificates/X509Pal.macOS.cs	Removes DSA public key decoding logic
src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/X509Certificates/X509CertificateLoader.macOS.cs	Removes DSA key handling and import logic
src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/X509Certificates/AppleCertificatePal.Keys.macOS.cs	Replaces DSA implementation with PlatformNotSupportedException
src/libraries/System.Security.Cryptography/src/System.Security.Cryptography.csproj	Removes DSA SecurityTransforms source files and adds NotSupported implementation
Multiple test files	Updates test attributes from SkipOnPlatform to ConditionalFact/ConditionalTheory using PlatformSupport.IsDSASupported
src/libraries/Common/tests/System/Security/Cryptography/PlatformSupport.cs	Adds IsDSASupported property excluding Apple platforms
src/libraries/Common/src/System/Security/Cryptography/DSASecurityTransforms.*.cs	Complete removal of DSA SecurityTransforms implementation files

[dotnet-policy-service]

Added needs-breaking-change-doc-created label because this PR has the breaking-change label.

When you commit this breaking change:

1. Create and link to this PR and the issue a matching issue in the dotnet/docs repo using the breaking change documentation template, then remove this needs-breaking-change-doc-created label.
2. Ask a committer to mail the .NET Breaking Change Notification DL.

Tagging @dotnet/compat for awareness of the breaking change.

[vcsjones]

Breaking change doc: dotnet/docs#48201