[release/10.0] Detect if RSA-384 is supported on the platform

[vcsjones]

Backport of #120366 to release/10.0

/cc @bartonjs

Customer Impact

• Customer reported
• Found internally

This is test-only reaction to a change in Windows. Tests started failing in Windows because a Windows Update caused RSA with a 384-bit modulus to fail.

Regression

• Yes
• No

Testing

This is test only change. Verified failing tests started passing again.

Risk

None. Test-only change.

[Copilot]

Pull Request Overview

This PR backports a fix to detect RSA-384 support dynamically on platforms rather than relying on hardcoded Windows version checks. The change responds to a Windows Update that caused RSA with 384-bit modulus operations to fail, breaking existing tests.

Key changes:

• Replaces static Windows version-based RSA-384 support detection with dynamic runtime testing
• Centralizes RSA-384 support detection in a shared PlatformSupport class
• Updates all RSA provider test classes to use the new centralized detection method

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/libraries/Common/tests/System/Security/Cryptography/PlatformSupport.cs	Adds new IsRSA384Supported property with dynamic detection logic and test RSA-384 key parameters
src/libraries/System.Security.Cryptography/tests/DefaultRSAProvider.cs	Replaces hardcoded Windows version check with centralized platform support detection
src/libraries/System.Security.Cryptography.Csp/tests/RSACryptoServiceProviderProvider.cs	Updates from hardcoded true to use centralized platform support detection
src/libraries/System.Security.Cryptography.Cng/tests/RSACngProvider.cs	Replaces hardcoded Windows version check with centralized platform support detection

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[bartonjs]

@artl93 Test-only change

[artl93]

yup - test only change.