Improve error message when running on outdated Windows OS with incomplete CET support

[jkotas]

Fixes #110000

[Copilot]

Pull Request Overview

This PR improves the error messaging when running on outdated Windows OS versions that have incomplete CET support. It replaces an assertion with a runtime fatal error that provides a clear, user‐facing message.

• Replaced the assertion regarding shadow stacks with an explicit error check.
• Introduced a detailed error message prompting users to install available Windows updates.

Comments suppressed due to low confidence (1)

src/coreclr/vm/threads.cpp:7709

• The updated error message clearly informs users about incomplete CET support on Windows. Ensure that the message format and localization (if applicable) remain consistent with similar error messages elsewhere in the codebase, and consider referencing documentation for further guidance.

if (AreShadowStacksEnabled() && !UseSpecialUserModeApc())

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