Delete duplicate policy

[gewarren]

• Verified that the mention functionality is working, so delete the extra policy for System.Security.
• Also added new mentions for WPF and System.Runtime

(Hide whitespace changes in diff.)

[learn-build-service-prod]

Learn Build status updates of commit afa8b4a:

✅ Validation status: passed

File	Status	Preview URL	Details
.github/policies/mention-owners.yml	✅Succeeded

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel

[learn-build-service-prod]

Learn Build status updates of commit dc87108:

✅ Validation status: passed

File	Status	Preview URL	Details
.github/policies/mention-owners.yml	✅Succeeded

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel

[BillWagner]

This LGTM @gewarren

Let's